Don't try and clear non-text/file inputs

The existing code causes non `text`/`file` inputs to be cleared (eg `radio`), which results in a "Element must be user-editable in order to clear it" exception (see https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/clear).

The line is redundant too, since `text` inputs are handled by the preceeding case.

(On a related note, I don't know what Selenium2 does with the new HTML5 inputs (`email`, `tel` etc), the docs *say* `text` only, so this might affect them too.)