Remove \it and other oldfonts from math completion

Remove \it and other oldfonts from math completion

\it and the other oldfont commands are deprecated and the new forms
(e.g. \mathit) should be used instead. The old forms can lead to a
warning in TeX Live 2015 and an error in TeX Live 2016.

Note that with this commit only affects completion, so LyX still
supports displaying e.g. \it in math if the user enters it without
completion.

This is consistent with 24d01111.

Thanks to Guillaume for the patch.