In BP_Group_Extension::call_edit_screen_save(), don't save if DOING_AJAX

Previously, the settings save routine was running whenever the POST global was
populated. This meant running when handling an AJAX request sent via POST. But
the latter is incorrect behavior, since it disallows the use of AJAX on the
edit screen for anything other than saving the form. As a result, all AJAX
requests sent via POST on group edit screens were resulting in a failed nonce
check.

Plugins that wish to save group edit settings via AJAX should implement their
own nonce checks.

See #5052

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7251 cdf35c40-ae34-48e0-9cc9-0c9da1808c22