solenv/bin/pack_images.py no longer needs executable bit

...after 906a2e4f "Always execute pack_images.py
with the right Python interpreter" removed its shebang line

Change-Id: I39dfa9c57699594caad85e183d3ef90b9ecaa59b
Reviewed-on: https://gerrit.libreoffice.org/56872
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>