modules: trusted-firmware-m: Remove 'xtools' toolchain support

This commit removes the TF-M support for the `xtools` toolchain variant,
which has been deprecated since Zephyr v3.3.0 and now removed.

Note that `zephyr` toolchain variant must be used instead of `xtools` when
building with Zephyr SDK.

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