self-test: fix make clean

thuge-gen was forgotten.  Fix it by removing the duplication, so we don't
get too many repeats.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>