Deactivate MP6 on update to 3.8.

If left on, it introduces a lot of weird issues not easily diagnosed. MP6 will be updated before release to prevent itself from functioning 3.8+, but this will cut down on support requests by people who update core but not MP6, which was a fairly popular plugin.

fixes #26334.

Built from https://develop.svn.wordpress.org/trunk@26692


git-svn-id: http://core.svn.wordpress.org/trunk@26582 1a063a9b-81f0-0310-95a4-ce76da25c4cd