This guard was removed in https://github.com/ruby/ruby/pull/13497
on the justification that some GC may need to be notified even for
immediate.

But the two currently available GCs don't, and there are plenty
of assumtions GCs don't everywhere, notably in YJIT and ZJIT.

This optimization is also not so micro (but not huge either).
I routinely see 1-2% wasted there on micro-benchmarks.

So perhaps if in the future we actually need this, it might make
sense to introduce a way for GCs to declare that as an option,
but in the meantime it's extra overhead with little gain.

Some GC implementations want to always know when an object is written to,
even if the written value is a special constant. Checking special constants
in rb_obj_written was a micro-optimization that made assumptions about
the GC implementation.

- ISO C forbids conversion of function pointer to object pointer type
- ISO C forbids conversion of object pointer to function pointer type

and declare it will be removed soon.

ddtrace is still referes the API and build was failed.
See https://github.com/DataDog/dd-trace-rb/pull/3578

Maybe threre are only few users of this C-API now so we can remove
it soon.

This function has been deprecated since Ruby 3.1, so we should remove it
for Ruby 3.4.

Instead of `RUBY_REFERENCES_START` and `RUBY_REFERENCES_END`, so that
auto-indent works well.

Also move `struct` so that `typedef`-ed names can be used.

This compile time flag was removed in https://github.com/ruby/ruby/pull/7313

This commit cleans up some related dead code.