bug 1419532: Delete dev emails with make clean
bug 1419532: Delete dev emails with make clean

Clear out the development emails with "make clean", which saves a small bit of
disk space (for me, 20 MB over 6+ months) and makes it easier to find the
latest email when debugging.

Elchi3/kuma,Elchi3/kuma,Elchi3/kuma,jwhitlock/kuma,SphinxKnight/kuma,Elchi3/kuma,jwhitlock/kuma,Elchi3/kuma,jwhitlock/kuma,SphinxKnight/kuma,escattone/kuma,a2sheppy/kuma,SphinxKnight/kuma,a2sheppy/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,escattone/kuma,mozilla/kuma,SphinxKnight/kuma,a2sheppy/kuma,a2sheppy/kuma,mozilla/kuma,SphinxKnight/kuma,mozilla/kuma,mozilla/kuma,escattone/kuma,jwhitlock/kuma,jwhitlock/kuma