cmake: toolchain: Remove deprecated 'xtools' toolchain variant

The `xtools` toolchain variant, which was originally introduced to be used
with the Crosstool-NG-based Zephyr SDK toolchains and has been replaced by
the `zephyr` toolchain variant, has been marked as deprecated since Zephyr
v3.3.0.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>