x86: Remove ARCH_CET_LEGACY_BITMAP [BZ #25397]

Since legacy bitmap doesn't cover jitted code generated by legacy JIT
engine, it isn't very useful.  This patch removes ARCH_CET_LEGACY_BITMAP
and treats indirect branch tracking similar to shadow stack by removing
legacy bitmap support.

Tested on CET Linux/x86-64 and non-CET Linux/x86-64.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>