Eliminate find_pc_partial_function_gnu_ifunc

Not used anywhere any longer.

If this is ever reinstated, note that this case:

	  cache_pc_function_is_gnu_ifunc = TYPE_GNU_IFUNC (SYMBOL_TYPE (f));

was incorrect in that regular symbols never have type marked as GNU
ifunc type, only minimal symbols.  At some point I had some fix that
checking the matching minsym here.  But in the end I ended up just
eliminating need for this function, so that fix was not necessary.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* blockframe.c (cache_pc_function_is_gnu_ifunc): Delete.  Remove
	all references.
	(find_pc_partial_function_gnu_ifunc): Rename to ...
	(find_pc_partial_function): ... this, and remove references to
	'is_gnu_ifunc_p'.
	(find_pc_partial_function): Delete old implementation.
	* symtab.h (find_pc_partial_function_gnu_ifunc): Delete.