Remove LLVM attribute removal

[erikdesjardins]

This was necessary before, because declare_raw_fn would always apply
the default optimization attributes to every declared function.
Then attributes::from_fn_attrs would have to remove the default
attributes in the case of, e.g. #[optimize(speed)] in a -Os build.
(see src/test/codegen/optimize-attr-1.rs)

However, every relevant callsite of declare_raw_fn (i.e. where we
actually generate code for the function, and not e.g. a call to an
intrinsic, where optimization attributes don't [?] matter)
calls from_fn_attrs, so we can remove the attribute setting
from declare_raw_fn, and rely on from_fn_attrs to apply the correct
attributes all at once.

r? @ghost (blocked on #94221)
@rustbot label S-blocked