boards: Remove 'xtools' toolchain variant references

This commit removes all references to the `xtools` toolchain variant in the
board YAML files.

Note that the `xtools` toolchain variant has been deprecated since Zephyr
v3.3.0 and now removed.

The removal process was automated using the following command line:

  git grep -l xtools -- boards/*.{yml,yaml} | \
  xargs -n 1 -P $(nproc) \
  yq -i 'del(.toolchain[] | select(. == "xtools"))'

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