beaglebone-yocto: remove redundant XSERVER assignment

The default XSERVER value is good enough for this BSP, so we don't need
to set it explicitly.

(From meta-yocto rev: a0077d9132d1e86f97fa3d4a4607b008622aa17e)

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

meta-yocto-bsp/conf/machine/beaglebone-yocto.conf

@@ -3,9 +3,6 @@
 #@DESCRIPTION: Reference machine configuration for http://beagleboard.org/bone and http://beagleboard.org/black boards
 
 PREFERRED_PROVIDER_virtual/xserver ?= "xserver-xorg"
-XSERVER ?= "xserver-xorg \
-           xf86-video-modesetting \
-           "
 
 MACHINE_EXTRA_RRECOMMENDS = "kernel-modules kernel-devicetree"