DIRDEPS_BUILD: Don't hook 'beforebuild' into 'all' in MAKELEVEL 0.

This was a mistake in r295641, but is effectively prevented from
being a problem due to r296699 preventing 'make all' from being
usable at MAKELEVEL 0 for DIRDEPS_BUILD.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon