mediawiki.mixins: Remove -o and -moz vendor prefixes for transition

-moz is needed for Firefox <= 15.0, -o is needed for Opera <= 12.0 [1].
Usage of those browsers is negligible [2].

[1] http://caniuse.com/css-transitions
[2] http://stats.wikimedia.org/wikimedia/squids/SquidReportClients.htm

Change-Id: I43bcf1efacb87ab61b78140fe85bd846d791b54a
Merge "mediawiki.mixins: Remove -o and -moz vendor prefixes for transition"