Remove call to isMounted

This was introduced with mui#3287 as migration from
isMounted happened after this PR was opened.

Author: alitaheri

src/TextField/TextField.jsx

@@ -365,7 +365,7 @@ const TextField = React.createClass({
   },
 
   select() {
-    if (this.isMounted()) this._getInputNode().select();
+    if (this.input) this._getInputNode().select();
   },
 
   getValue() {