remove insecure reattach check

See #2013; given that plaintext is deprecated now, it seems like there is no
added value from continuing to police this.

irc/client_lookup_set.go

@@ -195,15 +195,6 @@ func (clients *ClientManager) SetNick(client *Client, session *Session, newNick
 			dryRun || session == nil {
 			return "", errNicknameInUse, false
 		}
-		// check TLS modes
-		if client.HasMode(modes.TLS) != currentClient.HasMode(modes.TLS) {
-			if useAccountName {
-				// #955: this is fatal because they can't fix it by trying a different nick
-				return "", errInsecureReattach, false
-			} else {
-				return "", errNicknameInUse, false
-			}
-		}
 		reattachSuccessful, numSessions, lastSeen, wasAway, nowAway := currentClient.AddSession(session)
 		if !reattachSuccessful {
 			return "", errNicknameInUse, false

irc/server.go

@@ -359,10 +359,7 @@ func (server *Server) tryRegister(c *Client, session *Session) (exiting bool) {
 	rb := NewResponseBuffer(session)
 	nickError := performNickChange(server, c, c, session, c.preregNick, rb)
 	rb.Send(true)
-	if nickError == errInsecureReattach {
-		c.Quit(c.t("You can't mix secure and insecure connections to this account"), nil)
-		return true
-	} else if nickError != nil {
+	if nickError != nil {
 		c.preregNick = ""
 		return false
 	}