drm/i915/guc: drop negative doorbell alloc selftest

The test requires driver tweaks to avoid causing error messages
on intentionally-triggered errors and to stop accessing non
existing register. However, this is a pure GuC FW interface test
and should be covered by FW validation, so it isn't really worth
tweaking the driver for it and we're better off dropping it instead.

Testing the driver running out of doorbells is already covered by
igt_guc_doorbells

Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181018004610.22895-1-daniele.ceraolospurio@intel.com