Make it more obvious how to clear the Test Set filter on an Execution

1. Create a Test Execution issue
2. Add two Test Sets to the Execution
3. Click the Test Set filter on the Execution
4. Select one of the Sets from the list - the tests shown are filtered according to the selected set

Now I want to clear the filter I chose previously - click the Test Set filter again and note that there is an All entry in the list. This suggests that if I click it, I will be shown all tests (or maybe all Sets?). This is not the case, the All option cannot be selected by clicking it. As it does nothing, it should probably be removed?

It took several of our users some time to realise that you need to click into the filter and then press Delete on the keyboard to clear the filter. It would be more user-friendly if there was an "x" that could be clicked to remove the current filter. Attached shows an example of how this is used for the Fix Versions list in the standard JIRA UI.