ino64: Remove 'forward compat' code for this

Forward compatibility code was added for running newer ino64 binaries on
older kernels as a transition aide. Now that ino64 has been in the tree
6 years, this code is no longer useful and should have been removed long
ago. Remove it now. Should be no user-visible changes at this point as
all the 'upgrade' scenarios it was intended for are long since past.

Also need to remove this stuff from rtld since the _foo versions
no longer exist.

Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D42382