Disallow and hide user removal (as opposed to deletion) for single site...

Disallow and hide user removal (as opposed to deletion) for single site installs.  Removal is a multisite concept.  Props nacin. fixes #16501 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@17439 1a063a9b-81f0-0310-95a4-ce76da25c4cd