Revert "Add maxlength of 6 for time correction in preferences"

As noted by Bartosz, System|60 is also valid (apparently also 
ZoneInfo|something, but not sure what can go after ZoneInfo), and the 
maxlength interferes with this.

Maybe eventually we could revisit and use input#pattern 
(http://www.w3.org/TR/html51/forms.html#the-pattern-attribute).  That 
allows specifying a regex, which could have different length limits 
depending whether it started with a number (AFAICT, the 6 length is valid 
if it starts with a digit).

But for now, that is not urgent, and we need to fix the regression.

This reverts commit da470ce005652774049d346142b9509e89b31c6b.

Bug: 70405
Change-Id: Ib585c76688284dab78e85fb355785c06d0c3e353
Merge "Revert "Add maxlength of 6 for time correction in preferences""