jquery.textSelection: Remove unnecessary checks
Some of the commands were wrapped in unnecessary checks (which were
also inconsistent between the different commands):
* Checking if this element is hidden. I'm guessing this was a
workaround for some IE 6 bugs where doing things to the selection
of hidden elements would break horribly. In any case, this should
not be this code's responsibility.
* Checking for undefined this.selectionStart. I probably should have
removed these in
1430ca85dcf9cb3f77a92bee6a915473e465f649. The only
remaining case for them would be if .textSelection() is called on
something that is not a text input element, which should not happen.
Change-Id: Ib09e64399921fe3861efd4877c6ecb2f900aef09