Bluetooth: Mesh: Moved composition data out of mesh shell

The mesh shell module owns the composition data for the shell
application, which makes it impossible to use it outside of the
application itself. To support the shell as a generic debugging
component that can be added to any application, we have moved the
composition data out of shell.c, and into the application.

Signed-off-by: Ingar Kulbrandstad <ingar.kulbrandstad@nordicsemi.no>