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.