Only remove build directories before executing, not after

To help in the case where a build fails and the user wants
to investigate why it fails it is nice to only delete the
required directories *before* building and not before and
after (even under failures).

Fixes bug 1355442

Change-Id: I0998fb0c8b80f81ac78075f9e6d0d9c2f65a0eba
Merge "Only remove build directories before executing, not after"