dim: stop generating drm-intel-testing requests

Since the drm-intel-testing branch was introduced, we've completely
shifted from testing pull requests to pre-merge patch testing and
continuous testing. The drm-intel-testing branch and testing request has
outlived its usefulness. Remove it.

We still leave the workflow for tagging and making pull requests as they
are for now.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>