Remove DbConfigTask.

This task does not work well with either the new world of configuration
or the non-interactive features of bake. Removing it feels like the best
option right now.
Update BakeShell.

Update bakeshell to use new task code and simplified internals.
Update bake menu.
Cleanup after removing DbConfigTask.
Fix build failures.
Merge pull request #3190 from cakephp/3.0-dbconfig

3.0 Remove DbConfig task and update BakeShell