refactor(material/dialog): dialog name not read by sr on mac with chrome or firefox

Cleans up unused variables and comments.

Fixes b/274674581

Author: essjay05

src/material/dialog/dialog-container.ts

@@ -145,7 +145,6 @@ export class MatDialogContainer
     } else if (!os && /linux/.test(userAgent)) {
       os = 'linux';
     }
-    // const platform = window.navigator.userAgent.platform
     return os;
   };
 
@@ -175,6 +174,7 @@ export class MatDialogContainer
     }
     return;
   };
+
   ngOnInit() {
     this._setAriaLabel();
   }