gdb: remove lookup_bound_minimal_symbol

Now that lookup_minimal_symbol has default values for sfile and objf,
calling lookup_bound_minimal_symbol is identical to calling
lookup_minimal_symbol without sfile and objf.  Remove
lookup_bound_minimal_symbol, replace call sites with
lookup_minimal_symbol.

Change-Id: I0a420fb56de1de8bee8a7303228c9e4546e3577b
Reviewed-by: Keith Seitz <keiths@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>