Double wiki name prefix in group members listings

This can be reproduced in a XEM environment, by managing global groups or viewing a local group page.

The error seems to be caused by some bad JSON generation code in getgroupmembers.vm.

The following snippet appears twice and is incorrect:

"$xwiki.getUserName($m, false)#if($hasAdmin || $isAdvancedUser) (#if($wikiname != 'local')$wikiname:#end$m)#end"

The test #if($wikiname != 'local')$wikiname:#end is obviously incorrect and seems unnecessary as $m already contains the fullname of the document(including the wiki name).