gdb: remove get_current_regcache

gdb: remove get_current_regcache

Remove get_current_regcache, inlining the call to get_thread_regcache in
callers.  When possible, pass the right thread_info object known from
the local context.  Otherwise, fall back to passing `inferior_thread ()`.

This makes the reference to global context bubble up one level, a small
step towards the long term goal of reducing the number of references to
global context (or rather, moving those references as close as possible
to the top of the call tree).

No behavior change expected.

Change-Id: Ifa6980c88825d803ea586546b6b4c633c33be8d6