Remove the hammer used to remove *.debug files in the
resulting ISO image userland.

Explicitly set MK_DEBUG_FILES=no, which overrides the
WITH_DEBUG_FILES=1 and WITHOUT_DEBUG_FILES=1 collisions
previously experienced.

This change allows us to create the {base,kernel}_debug.txz
distributions without accidentally installing the *.debug
files on the medium itself.

Inspired by: jmmv
Tested on: projects/release-debugdist@r267344
Sponsored by: The FreeBSD Foundation