Bug 1291667 - Removed unused variant 'MozNone' from StyleUserSelect enum class; r=heycam,manishearth

MozReview-Commit-ID: 53dJd2GYUdt

--HG--
extra : rebase_source : 406d58b113d3044eb9f48b02fdd0168494c22847

layout/style/nsStyleConsts.h

@@ -143,7 +143,6 @@ enum class StyleUserSelect : uint8_t {
   TriState,
   Auto,     // internal value - please use nsFrame::IsSelectable()
   MozAll,   // force selection of all children, unless an ancestor has NONE set - bug 48096
-  MozNone,  // Like NONE, but doesn't change selection behavior for descendants whose user-select is not AUTO.
   MozText,  // Like TEXT, except that it won't get overridden by ancestors having ALL.
 };