DefaultSettings.php: Remove $wgPasswordSenderName

It doesn't do anything.

Follow-up to Ibfd28cd181365c8c0b5f3e8ffe8f5de8c89844a3.

Change-Id: Id20e4ec1cdae94fadb278146cd72d5a9b247dccf
Merge "DefaultSettings.php: Remove $wgPasswordSenderName"