Update the class name on css to match the component (#15016)

We must not display the SF chat bubble on the mobile version. We handled
this during the migration from Zendesk to SF, but our css rule specifies
a class that's defined on SF stylesheet, which became outdated after the
recent upgrade.

Changes in this PR point update the class name to meet the current
definition.

src/Components/SalesforceWrapper.tsx

@@ -9,20 +9,10 @@ export const SalesforceWrapper: React.FC<{}> = _props => {
   useAppendStylesheet({
     id: "salesforce-chat-styles",
     body: `
-      .embeddedServiceHelpButton .helpButton .uiButton {
-        background: #000000; /* colors.black100 */
-        font-family: "ll-unica77", "Helvetica Neue", Helvetica, Arial, sans-serif; /* fonts.sans */
-        box-shadow: none;
-      }
-    
-      .embeddedServiceHelpButton .helpButton .uiButton:focus {
-        background: #1023D7; /* colors.blue100 */
-      }
-      
       /* Make sure it doesn't render when loaded on desktop and then the window is resized */
       @media (max-width: 767px) { /* mediaQueries.xs */
-        .embeddedServiceHelpButton .helpButton .uiButton {
-          display: none;
+        .embeddedMessagingConversationButton {
+          display: none !important;
         }
       }
     `,