encodings: clear postinst script

Postinst script from xorg-font-common.inc doesn't apply to this recipe.
So clear the postinst script of encodings.

(From OE-Core rev: ba94c908)

Signed-off-by: Kai Kang <kai.kang@windriver.com>
(cherry picked from commit 99ae6dbb)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>