Remove $wgServerName. Its only usage was for {{servername}}, and needed to be kept...

Remove $wgServerName. Its only usage was for {{servername}}, and needed to be kept in sync with $wgServer in LocalSettings.
None of the 3 globals based on it changed if you set it in LocalSettings.

Note that all those !isset( $wgServerName ) in ApiTests were useless, since if not in LocalSettings it would be 'localhost', not null (as still are those !isset( $wgServer )).