fix warning and untranslate "about:blank"

Fixing this warning:
`...\MuseScore\mscore\prefsdialog.ui:-1: Warning: The name 'gridLayout6' (QGridLayout) is already in use, defaulting to 'gridLayout61'.`

No sure whether that 'untranslate' really works, I edited the ui file manually, the Qt Creator GUI didn't have a 'translate' checkbox for that string.