Remove unused 'use' statements

As part of #2926791: All coding standard fixes , here's a patch to remove 45 unused 'use' statements, as reported in the current branch test https://www.drupal.org/pift-ci-job/820001

I went through each file and verified there was no usage of the removed classes/interfaces referenced by these 'use' statements, and I ran the Flag tests locally to ensure everything still works. The patch tests results will tell us if I missed any ... This patch should be fairly easy to review because you can easily see if there are any out-of-scope changes, and because it removes 'use' statements there will be test failures if the patch removed something it shouldn't have.