Revert "Adorn __call_once_proxy with `inline` and `_LIBCPP_INLINE_VISIBILITY`"

While this change didn't really hurt, it does lead to spurious
warnings about not being able to override weak symbols if you end up
linking objects built with this change to ones built without it.
Furthermore, since __call_once_proxy is called indirectly anyway it
doesn't actually inline ever.

Longer term, it would probably make sense to give this symbol internal
visibility instead.

This reverts r291497

llvm-svn: 293581