fix JS errors in saving preferences - for user's display (including custom theme vector) and advanced settings (#6764)

webapp/components/user_settings/user_settings_advanced.jsx

@@ -137,9 +137,6 @@ export default class AdvancedSettingsDisplay extends React.Component {
             preferences,
             () => {
                 this.updateSection('');
-            },
-            (err) => {
-                this.setState({serverError: err.message});
             }
         );
     }

webapp/components/user_settings/user_settings_display.jsx

@@ -94,9 +94,6 @@ export default class UserSettingsDisplay extends React.Component {
         savePreferences([timePreference, namePreference, fontPreference, channelDisplayModePreference, messageDisplayPreference, collapseDisplayPreference],
             () => {
                 this.updateSection('');
-            },
-            (err) => {
-                this.setState({serverError: err.message});
             }
         );
     }