refactor: remove unused files; improve build UX

[lesshonor]

The Kconfig files were mistakenly placed under boards/shields as part of #224.

Willing to break out the makefile change into a separate PR RFC-style, but IMO the "Did not find ${shield}" message here is actively unhelpful:

• Users mistakenly latch onto it as a reason for build failures because it is the first "error" they see written in plain English, and separated from the rest of the build output by a blank line
• It displays for all out-of-tree shields
• It displays for in-tree shields if they are splits, or built with an out-of-tree module/config repo (e.g. my_shield nice_view_adapter nice_view)
• The build will fail within seconds of the warning if any shield specified is actually missing
• Per refactor: Hook into CMake loading in a better spot. #1989, it should not be WARN-worthy that a shield does not exist in the main repo when (emphasis mine):

  [An independently managed Zephyr module repository] is the preferred way to handle boards and shields moving forward in ZMK [...] ZMK does have a collection of boards/shields in the ZMK main repository, which are planned to be phased out [...]

[caksoylar]

I like these changes. Kconfig files do seem look the result of a mistake to me, so makes sense to remove them.

I also agree that that warning has caused a lot of confusion. I think it would be great to remove that (thanks for digging up the cause!).

[petejohanson]

Thanks. Agreed on all points.