Convert lookup_symbol_in_objfile

This converts lookup_symbol_in_objfile to the callback approach by
removing the call to lookup_symbol_in_objfile_symtabs.  (The latter is
not removed as there are still other callers.)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16994
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16998
Acked-By: Simon Marchi <simon.marchi@efficios.com>