graphics/nvidia-drm-kmod: drop conditional possibly applies nonexistent patch after commit ports 3bcc752b87a2

[junchoon_dec.sakura.ne.jp]

Summary

On commit ports 3bcc752b87a2, supports for 15.0-RELEASE are dropped
in all ports.

Afterwards, graphics/nvidia-drm-kmod/Makefile.common still has
a conditional that could attempt to apply deleted patch.

Remove the whole affected conditional to avoid potential breakage
on attempting to override version.

Not bumping PORTREVISION, as this should NOT affect official
pkg builds.

PR: 299111

[junchoon_dec.sakura.ne.jp]

Note that another conditional (the one next to the removed conditional) would better dropped,
but intentionally excluded here, as it isn't affected by commit ports 3bcc752b87a2.

Should I include the deletion here (with the patch conditionally applied), too?