Remove -moz-background-size from .background-size LESS mixin

This is only used by Firefox 3.6. Firefox 4 supports it without the
vendor prefix and Firefox <= 3.5 doesn't support it at all. Not worth
it.
https://developer.mozilla.org/en-US/docs/Web/CSS/background-size

Change-Id: Ib1e876f75a7f331cae0300914eea1e69858915bb
Merge "Remove -moz-background-size from .background-size LESS mixin"