jquery.textSelection: Remove incorrect browser hack for old Opera

jquery.textSelection: Remove incorrect browser hack for old Opera

This code appears to be incorrect, at least on Opera 12.18 on Windows 10,
where I just tested it. It causes the selection to be misaligned, rather
than correct it.

For example, the following code:

    $textbox.textSelection( 'encapsulateSelection', { peri: 'foo\nfoo' } );

Causes the selection to extend one character past the inserted text with
the hack, and to be shown correctly without the hack.

It is theoretically possible that this hack was necessary for some other
operating system, but then it should have also been applied to 'insertText'.
Also, Opera 12 only has a usage share (barely) large enough to show up on
<https://analytics.wikimedia.org/dashboards/browsers/> on Windows (XP).

Change-Id: Ifee877c6202bcfc0fecb65220e34b4b39659dbeb