bind: Remove legacy python3 PACKAGECONFIG code

Python support was dropped upstream and removed in 8a9a5885 ("bind:
update 9.16.26 -> 9.18.1"), clean up the remaining pieces of python3 in
the recipe.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ee4e4eb1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>