Profiler UI doesn't let me remove the last samples track (to leave only a screenshot track)

I'd like to be able to upload a profile that consists of _only_ a screenshots track; but, that doesn't seem to be allowed by our UI right now. It lets me un-tick tracks one at a time, but the profiler won't let me hide the last one (to leave only the profiler track).

Sample profile:, which has a screenshots track and a sample track for "Parent Process":
https://share.firefox.dev/36jaxPR

In the context menu for Parent Process, the "Hide Parent Process" menu-item doesn't have any effect, nor does clicking the menu-item below it with a checkmark.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-359)