[muc] Remove stale comment from MultiUserChat.userHashLeft()

Set started with Smack 4.4 to set myRoomJid to null, introduced by
ab2822be3e ("muc: also set myRoomJid to null if we have left the
room"), however the code comment still states that we "do not reset
nickname here" (and myRoomJid contains the nickname as its
resourcepart).

Reported-by: Damian Minkov <damencho@jitsi.org>

smack-extensions/src/main/java/org/jivesoftware/smackx/muc/MultiUserChat.java

@@ -2201,9 +2201,6 @@ public class MultiUserChat {
      * Remove all callbacks and resources necessary when the user has left the room for some reason.
      */
     private synchronized void userHasLeft() {
-        // We do not reset nickname here, in case this method has been called erroneously, it should still be possible
-        // to call leave() in order to resync the state. And leave() requires the nickname to send the unsubscribe
-        // presence.
         occupantsMap.clear();
         myRoomJid = null;
         // Update the list of joined rooms