[Libomptarget] Remove handling of old ctor / dtor entries (#80153)

Summary:
A previous patch removed creating these entries in clang in favor of the
backend emitting a callable kernel and having the runtime call that if
present. The support for the old style was kept around in LLVM 18.0 but
now that we have forked to 19.0 we should remove the support.

The effect of this would be that an application linking against a newer
libomptarget that still had the old constructors will no longer be
called. In that case, they can either recompile or use the
`libomptarget.so.18` that comes with the previous release.