Function lookup_minimal_symbol_and_objfile iterates on all objfiles and
calls lookup_minimal_symbol for each of them, effectively searching in all
objfiles.  lookup_bound_minimal_symbol calls lookup_minimal_symbol with NULL,
which also effectively searches all objfiles.  AFAIK, they do exactly the same
thing, so we can get rid of one (and lookup_minimal_symbol_and_objfile happens
to be the most inefficient because it ends up n^2 on the number of objfiles).

Tested in both aarch64-linux-gnu and amd64-linux-gnu. No regressions.