drm/i915/vrr: drop window2_delay member from i915

The window2_delay member has been functionally unused (always set to 0)
since it was added in commit bb265dbd ("drm/i915/xelpd: Add VRR
guardband for VRR CTL"). Replace it with a FIXME comment.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/036955dc2c4c2d2b2d89555e473d91ce1be10395.1661346845.git.jani.nikula@intel.com