mhbuild.c: Don't use set_done() to unlink temporary files.

mhbuild.c: Don't use set_done() to unlink temporary files.

Because they're created with m_mktemp2(), the temporary files are
already in a list to be unlink(2)'d at exit(3), thanks to atexit(3), so
don't set_done() to attempt that twice.  Updates 6bc64765f.