dap-magik.el: dap-magik-path-mapping defcustom badly defined.

The type of the current definition of `dap-magik-path-mapping` defcustom is `'vector` but does not define its elements.

Opening the customize buffer for the dap-magik- group shows a `(mismatch)` on the value caused by this invalid defcustom definition.

I can't identify what it should really be, I did not find any code that attempts to use the value...
- Should this user-option be simply removed?
