fix(android): drop the unused kotlin-android plugin

[gabrieldonadel]

Problem

Android Gradle Plugin 9 ships built-in Kotlin support and enables it by default, so
AGP registers the kotlin extension itself. When a library also applies kotlin-android
explicitly, the two collide and configuration fails before anything compiles:

> Failed to apply plugin 'kotlin-android'.
   > Cannot add extension with name 'kotlin', as there is an extension already registered with that name.

The apply is unconditional in this file, so on an AGP 9 project this module cannot be
built at all.

Change

Remove the kotlin-android apply.

 apply plugin: 'com.android.library'
-apply plugin: 'kotlin-android'
 apply plugin: 'com.facebook.react'

Files changed:

• android/build.gradle

This PR originally guarded the apply behind a check for the kotlin extension. At
@jokerttu's suggestion it now removes the line instead, which is the better fix here:
the module has no Kotlin to compile, so the plugin was doing nothing on any AGP version.

Why removal is safe in this module

• android/src holds 33 .java files and no .kt files. The only Kotlin in the
  repository is under example/android/app, which is a separate Gradle project with its
  own build file and is unaffected.
• Android codegen emits Java here — codegenConfig sets android.javaPackageName — so
  the generated/java and generated/jni directories added to sourceSets.main.java.srcDirs
  contain no Kotlin either.
• Nothing else in android/build.gradle depends on the Kotlin plugin: no kotlinOptions
  block, no Kotlin stdlib dependency, no kotlin.srcDirs.

Because there are no Kotlin sources, this is a no-op on AGP 8 as well as a fix on AGP 9 —
compileDebugKotlin had nothing to do before and does not exist now.

What I verified, and what I did not

• Inventoried the module's sources and checked every Kotlin-dependent construct listed
  above.
• The changed file passes a Groovy Phases.CONVERSION syntax check.
• Not run: this repo's own CI or the example app. A CI run is the real confirmation
  and I could not do that from outside.

Found while sweeping popular React Native libraries for AGP 9 new-DSL compatibility,
where this collision was by far the most common blocker. Most libraries do have Kotlin
sources and need the guard; this one does not, and is better off without the plugin.

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[gabrieldonadel]

@googlebot I signed it!

[jokerttu]

Thanks for the report; the collision is real.
I'd rather fix it at the root than guard it, though.
See the suggestion.

[gabrieldonadel]

Agreed — you're right, and removing it is the better fix here. Applied in ee9db75; the
PR is now a one-line deletion.

I checked the module before taking it, since the guard is the right answer in most of the
libraries in this sweep and I wanted to be sure this one is genuinely different:

• android/src has 33 .java files and no .kt files. The only Kotlin in the repo is
  under example/android/app, which is a separate Gradle project with its own build file.
• codegenConfig sets android.javaPackageName, so codegen emits Java — the
  generated/java and generated/jni directories added to sourceSets.main.java.srcDirs
  bring no Kotlin in either.
• Nothing else in android/build.gradle needs the plugin: no kotlinOptions, no Kotlin
  stdlib dependency, no kotlin.srcDirs.

So the plugin was a no-op on AGP 8 too, not just redundant on AGP 9. Title and description
updated to describe the removal rather than the guard.

Worth flagging one thing for whenever you do move this module to Kotlin: under AGP 9,
built-in Kotlin does not read java.srcDirs for Kotlin sources. The generated/java and
generated/jni entries in your sourceSets block would need mirroring into
kotlin.srcDirs at that point, or the files get dropped silently and the build fails with
Unresolved reference. Not an issue today with no Kotlin in the module.