[Feature] CMB column example

[Mte90]

So CMB2 has a parameter that let you to show the field (also with a custom callback) in the post type table list.
This means that we can remove the CPT_Columns library or in case add the CMB example to use that parameter.

https://github.com/CMB2/CMB2/wiki/Field-Parameters#column