Bugfixes in sales item pricing

I fix some bugs in the client-side sales item pricing:

- make rows removable after changing the type of reference rows to
  another value than "relativeToPos".
- function _getReferrers() now returns only referrers if their type is
  set to "relativeToPos"
- update the relToPos input control values and the visible
  relative-to-pos position marker if a row before is removed
- remove the title text which is set at the delete button of
  non-removable rows because it's obsolete