gdb: remove read_frame_register_value's frame parameter

gdb: remove read_frame_register_value's frame parameter

By now, all register struct values should have a valid next frame id
(assuming they are created using value::allocate_register or
value::allocate_register_lazy), so there should be no need to pass a
frame alongside the value to read_frame_register_value.  Remove the
frame parameter and adjust read_frame_register_value accordingly.

While at it, make read_frame_register_value static, it's only used in
findvar.c.

Change-Id: I118959ef8c628499297c67810916e8ba9934bfac