Fixes a Grid issue that breaks sorting. (#10999)

Fixes a Grid issue that breaks sorting. (#10999)

* Fixes a Grid issue that breaks sorting.

When multi-select has been enabled and disabled a number of times, sorting on the first column in a Grid is broken.
Unregistering a click handler on the header solves the problem.

* Added a test checking that sorting of a grid column still works after having enabled and then disabled multi select mode.
The grid is configured with an extra header row containing a text box since it triggers an exception if the header click handler for the multi select column is not removed.

* Remove an unused import that was added by mistake