drm/xe: Remove dead clock code

xe_gt_clock_cycles_to_ns() is not called from anywhere after PMU
handling was removed in commit 90a8b23f ("drm/xe/pmu: Remove PMU
from Xe till uapi is finalized"). Drop it.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408151312.2100304-1-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>