Remove hashtab_obstack_allocate

I think that hashtabs should never be obstack-allocated.  In the past
this was convenient sometimes, because any new data structure needed a
corresponding cleanup.  However, with the switch to C++, resource
management has become much simpler; for example, a local variable can
simply be of type htab_up rather than hashtab_t, and the problem is
solved.

This patch removes hashtab_obstack_allocate to try to prevent this
anti-pattern from being used again.

gdbsupport/gdb-hashtab.h

@@ -42,9 +42,4 @@ htab_delete_entry (void *ptr)
   delete (T *) ptr;
 }
 
-/* Allocation and deallocation functions for the libiberty hash table
-   which use obstacks.  */
-void *hashtab_obstack_allocate (void *data, size_t size, size_t count);
-void dummy_obstack_deallocate (void *object, void *data);
-
 #endif /* GDBSUPPORT_GDB_HASHTAB_H */