acrn.conf: drop acrn machine configuration

zephyr can be build for 'acrn' with following configuration:

MACHINE = "intel-x86-64"
ZEPHYR_BOARD = "acrn"

Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>

conf/machine/acrn.conf

-#@TYPE: Machine
-#@NAME: acrn
-#@DESCRIPTION: Machine for Zephyr BOARD acrn
-
-require conf/machine/include/qemu.inc
-require conf/machine/include/tune-corei7-common.inc
-ZEPHYR_INHERIT_CLASSES += "zephyr-qemuboot"
-
-ARCH_acrn = "x86"