Issue 4384: Remove convert-ly rule \shiftOff -> \undo\shiftOn

Issue 4384: Remove convert-ly rule \shiftOff -> \undo\shiftOn

For the rationale, see
<URL:https://codereview.appspot.com/190500043/#msg1>

The presence of the rule will cause similarly surprising effects to
users as it would to the current state of documentation translations.

Even if it were properly mentioned in the diagnostic description of
the conversion rule, people would rarely look closely enough to
notice.