gdb: remove regcache::target

gdb: remove regcache::target

The regcache class takes a process_stratum_target and then exposes it
through regcache::target.  But it doesn't use it itself, suggesting it
doesn't really make sense to put it there.  The only user of
regcache::target is record_btrace_target::fetch_registers, but it might
as well just get it from the current target stack.  This simplifies a
little bit a patch later in this series.

Change-Id: I8878d875805681c77f469ac1a2bf3a508559a62d
Reviewed-By: Pedro Alves <pedro@palves.net>