graphics/nvidia-drm-[510|515|61|66]-kmod[-devel]: Drop excessive BUILD_DEPENDS
As per request from Baptiste Daroussin via Matrix, drop BUILD_DEPENDS
line from Makefile in each master ports.
This is to attempt fixing builds on FreeBSD[-ports]-kmods repo builder.
As corresponding graphics/drm-*-kmod ports are not needed to be
installed on build, it should be no harm.
What's needed is the distfile of corresponding graphics/drm-*-kmod, with
including Makefile.version of each corresponding graphics/drm-*-kmod.
Reported by: bapt
Differential Revision: https://reviews.freebsd.org/D53136