libweston: unexport weston_{pointer,keyboard,touch}_{create,destroy}()

We have weston_seat_{init,release}_{pointer,keyboard,touch}() as the
backend-facing API. There is no need to expose the create/destroy
functions which have been for internal use only for quite a long time.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
v1 Tested-by: Matt Hoosier <matt.hoosier@gmail.com>