jquery.textSelection: Remove final remnants of WikiEditor iframe support

jquery.textSelection: Remove final remnants of WikiEditor iframe support

After WikiEditor's experimental iframe mode was removed in
952d6b9d8fc1d4ac606b6f8d0a3341ba7ec27e02, the code here was mostly
removed in 62b4198859e4ac7cf41775dc8bc943272e3ddfc3, but this little
stub remained to support CodeEditor (it piggy-backed on that feature
to support textSelection() overrides). That was also cleaned up
afterwards in 37228b257896e0a36816a61543ed5809e8ad6596, but everyone
forgot about this little remnant.

This code was not doing anything, as nothing sets the 'context.$iframe'
property anymore, in any extension.

Change-Id: I0ffe2405df7d6aaecc51b06ba6c7fea5fd8380b4