drm/xe: Remove XE_GUC_CT_SELFTEST

XE_GUC_CT_SELFTEST enabled a debugfs entry to which ran a very simple
selftest ensuring the GuC CT code worked. This was added before the
kunit framework was available and before submissions were working too.
This test isn't worth porting over to the kunit frame as if the GuC CT
didn't work, literally almost nothing would work so just remove this.

Suggested-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>