Remove unused message 'resetpass'

This was (unintentionally?) replaced by 'changepassword' in r86280
(1a80d4637002), which renamed the special page from Special:ResetPass
to Special:ChangePassword.

'changepassword' was previously used in Special:Preferences until
r49790 (155ddf6de4a2) for a heading directly above the link to
Special:ResetPass.

I moved messages 'changepassword' and 'changepassword-summary' to
the correct blocks in MessagesEn.php and messages.inc and also
updated message documentation that referred to Special:ResetPass.

Change-Id: Ie5cbda5538f53c8e8d84c5e9d4d818fd745ff46c
Merge "Remove unused message 'resetpass'"