mesa: remove atom-pc specific overrides

These bbappends were used so that atom-pc would prefer mesa-dri over mesa-xlib.
We don't have mesa-xlib anymore, so these are not required.

(From meta-yocto rev: bf7e83a980e9df664d79c94fff31189b5abb04ca)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-yocto-bsp/recipes-graphics/mesa/mesa-dri_8.0.4.bbappend

@@ -1,3 +0,0 @@
-# Atom PCs have DRI support so use mesa-dri by default
-DEFAULT_PREFERENCE_atom-pc = "2"
-

meta-yocto-bsp/recipes-graphics/mesa/mesa-dri_git.bbappend

@@ -1,3 +0,0 @@
-# Atom PCs have DRI support so use mesa-dri by default
-DEFAULT_PREFERENCE_atom-pc = "1"
-