jquery.textSelection: Remove code supporting IE 6 selection APIs

Removing all the code that uses the document.selection and
document.body.createTextRange() APIs, or that only runs if
these APIs are present (workarounds for old IE bugs).

These APIs were supported by Internet Explorer up to version 9
and by Opera up to version 10.5. More recent versions of both
browsers only support the standard selection APIs.

Bug: T186005
Change-Id: I32dc5a16b03cf1c2ea44f6c7361447607ccc7e62