Remove tabindex from Special:Block

There is a autofocus on the Target field, which sets the start of
tabbing and than all the other fields are tabbed in the correct native
order. No need for extra tabindex order.
Tested with IE11 and FF28

Bug: 61257
Change-Id: Ic0d7365447733a23fa87c1d9b91c0f4f0f61bb39