gdb, hppa: remove unused hppa_lookup_stub_minimal_symbol

I stumbled over this while reviewing all objfiles traversals with regards
to impact of linker namespaces.

Recursive grep only finds two occurrences of hppa_lookup_stub_minimal_symbol:
  - the declaration in hppa-tdep.h.
  - the definition in hppa-tdep.c.

There appear to be no calls to this function.  Remove it.