Textinput: Only _destroy() clearButton when present

Fixes gh-7567
OK, I totally misread your comment here. I thought you did not want me to let the exception bubble up to qunit and have the test fail that way. 

There is still a good reason to ensure that ```_destroyClear()``` does not get called. If a text input does not have a clear button, ```_destroyClear()``` should not get called, whether it throws an exception or not. So, I think the second assertion *is* justified.

If you accept this line of reasoning, I'll restore this test and delete the one below.