xserver-xorg: remove xshmfence configure option

After the commit [1] introduced in openembedded-core layer,
some configure options is't carried over include xshmfence
option, so remove the xshmfence configure option to silence
the below warning.
  WARNING: xserver-xorg-2_21.1.1-r0 do_configure: QA Issue: xserver-xorg: invalid PACKAGECONFIG: xshmfence [invalid-packageconfig]

[1] https://git.openembedded.org/openembedded-core/commit/?id=e05abd87ee5d23750c641d0129d9c83db68ee2e8

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>

recipes-graphics/xorg-xserver/xserver-xorg_%.bbappend

@@ -1,4 +1,4 @@
-OPENGL_PKGCONFIGS:rpi = "dri glx ${@bb.utils.contains('MACHINE_FEATURES', 'vc4graphics', 'dri3 xshmfence glamor', '', d)}"
+OPENGL_PKGCONFIGS:rpi = "dri glx ${@bb.utils.contains('MACHINE_FEATURES', 'vc4graphics', 'dri3 glamor', '', d)}"
 
 # when using userland graphic KHR/khrplatform.h is provided by userland but virtual/libgl is provided by mesa-gl where
 # we explicitly delete KHR/khrplatform.h since its already coming from userland package