binder: remove unused binder_alloc->buffer_free

binder: remove unused binder_alloc->buffer_free

The ->buffer_free member was introduced in the first revision of the
driver under staging but it appears like it was never actually used
according to git's history. Remove it from binder_alloc.

Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Acked-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20220829201254.1814484-6-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>