[bin/mb] Disable pump mode again, add todo comment

Recent builds don't invalidate everything all the time making ccache
useful, and with unified sources preprocessing is a much smaller chunk
of build time.

Clobber builds:
pump mode: 6m
non-pump:  6m15s

Clobber build with ccache (not possible with pump): 2m

Pulling central after a few days and clobbering: ~4m, thanks to ccache.

So pump mode is only helpful with complete clobber builds with no recent
cache, and only slightly. With non-unified builds it's probably still
significantly faster.

Ideally this would have a --distcc-pump option for building older trees
where it is still a huge improvement.