Remove isset() check for $wgLocalTZoffset in Language.php

Do not use isset() to check for null; furthermore this variable
can no longer be null at this point now, so the check is useless.

Also remove the initial defintion of $minDiff since this was the
only case where it could be not set afterwards and fix spacing
after the comment.

Change-Id: I6e62ce1217e6e8750acb411673903fc3bde57841
Merge "Remove isset() check for $wgLocalTZoffset in Language.php"