staging: board: Remove KZM9D board staging code

With the EMMA Mobile USB Gadget staging driver removed, there is no
longer any use for the EMEV2 KZM9D board staging code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Link: https://lore.kernel.org/r/f54fe56524e0266a3c705315f04870988912cfcf.1705932585.git.geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>