GUACAMOLE-839: Ensure all SSO and related mechanisms set Credentials username.

This is necessary to ensure the ${GUAC_USERNAME} token is properly
substituted in cases where SSO and SSO-like auth methods are used.

extensions/guacamole-auth-sso/modules/guacamole-auth-sso-cas/src/main/java/org/apache/guacamole/auth/cas/ticket/TicketValidationService.java

@@ -163,10 +163,6 @@ public class TicketValidationService {
         // Canonicalize username as lowercase
         username = username.toLowerCase();
 
-        // Update credentials with username provided by CAS for sake of
-        // ${GUAC_USERNAME} token
-        credentials.setUsername(username);
-
         // Retrieve password, attempt decryption, and set credentials.
         Object credObj = ticketAttrs.remove("credential");
         if (credObj != null) {