BUG Temporarily disable overwrite warning on HtmlEditorField "insert media".

Temporarily disable overwrite warning on HtmlEditorField "insert media" dialogue until this feature properly supports validation error recovery.

This fix mitigates the immediate issue raised in https://github.com/silverstripe/silverstripe-framework/issues/2079 that prevents the dialogue from working once a file overwrite notification occurs.

The major issue remains however, as this dialogue does not gracefully handle errors during upload of any sort. This is a more serious issue that will need to be resolved in the future.