xe_call_for_each_device() has been unused since commit 57ecead3
("drm/xe/tests: Convert xe_mocs live tests"). Remove it and the related
dev_to_xe_device_fn() and struct kunit_test_data.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/fa3bb23d005313c9797f557e1211fde09fcb59cc.1723458544.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>