Remove redundant Windows installer signing
Remove redundant Windows installer signing

The signing in `electron-package.sh` is no longer needed, since it's already
handled in the new sign script for `electron-builder`.

vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web,vector-im/vector-web