Remove duplication in file header.
Remove unused method.

This method is no longer useful and can be removed.
Extract a conventions trait.

Having a trait with some commonly used inflections/conventions makes the
Shell class a bit cleaner and contain less junk. Having a trait also
allows userland code and other parts of the framework to re-use this
piece.
Fix failing test.
Merge pull request #3204 from cakephp/3.0-shell-cleanup

3.0 shell cleanup