[INTERNAL] ensure that existing port mappings are deleted

At least if Eclipse and Saros terminate normally.

Change-Id: Id1b4ffd4028bc51c264b9499d733a53cdabc961b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1250
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>