Fix #1553

Remove subscript/subsript leftovers from SCEditor since we don't support
that with MyCodes and it causes problems with copypasting symbols.
https://github.com/mybb/mybb/issues/1553
`supscript` should be `superscript`
https://github.com/samclarke/SCEditor/blob/master/src/jquery.sceditor.js#L3513