Remove unused fixtures.

Kill off a pile of fixtures we are no longer using.

Refs #2345
Remove AclShellTest case.

The AclShell has been deleted for a while, remove the test case (which
was skipped) as well.
Merge pull request #2642 from cakephp/3.0-remove-fixtures

3.0 remove fixtures
Make error message for missing fixture easier to understand.

Telling both the class, fixture reference and test case will help
people solve problems a bit quicker.