drm/i915: Remove unused supported_crtc from intel_load_detect_pipe

... and the no longer relevant comment. The code ceased stealing a pipe
for load detection a long time ago.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>