jquery.textSelection: Remove unnecessary checks

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