cppcheck: redundantAssignment

Let's not forget the result of RegDeleteValueA call

Change-Id: I9458040bc3a8960ab740ee3c5211eaa9ba24b1aa
Reviewed-on: https://gerrit.libreoffice.org/7772
Tested-by: David Tardon <dtardon@redhat.com>
Reviewed-by: David Tardon <dtardon@redhat.com>