[C++] Remove ObjectWriter after JsonWriter migration

[Reranko05]

Describe the enhancement requested

The simdjson migration introduces JsonWriter as a replacement for ObjectWriter.

Once all ObjectWriter users have been migrated, the old implementation and its associated build configuration can be removed.

Additional context

This is a follow-up task for GH-35460. It depends on all remaining ObjectWriter users being migrated to JsonWriter.

Parent Issue: #35460

Component(s)

C++

[Reranko05]

@rok Please add this as sub-issue to #35460

[Reranko05]

take

[rok]

This issue lacks the usual closing message. @pitrou did you use the merge button on the PR 50691 instead of using the script? :) Or is something more nefarious going on?

[pitrou]

I'm almost sure I used the script, so I'm not sure what happened.