DIRDEPS_BUILD: Cleanup leftover WORLDTMP files.

This will force any existing objects to rebuild if their .meta
files reference files from WORLDTMP. This is a problem after
the UNIFIED_OBJDIR effort caused buildworld and DIRDEPS_BUILD
to share an OBJDIR. Without cleaning these files the
Makefile.depend files end up with odd tmp/legacy/... entries
since some bootstrap-tools linger from there and otherwise
don't rebuild. Removing the files causes anything having
used WORLDTMP to rebuild.

Sponsored by: Dell EMC Isilon