cmake: zephyr_module: Remove useless 'west_arg' variable

8cc71679 renamed this variable to lower
case to indicate it's only used locally however WEST_ARG is used as a
parameter of zephyr_module.py in CMakeLists.txt when
CONFIG_BUILD_OUTPUT_META is enabled.
This variable was used when west had some limitation. It does not
really make sense now, so it has been removed and content of west_arg
variable has been added by default.

Signed-off-by: Thomas Gagneret <tgagneret@witekio.com>