Do not delete ifwt if it exists

If ifwt exists in the work environment do not delete it.
It speeds up creating multiple offline installers in a row as it does
not download the extract installer-framework tools every time.

Change-Id: Id309f59cf80f18633cc6707102ec08ca61774a2f
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>