staging: Remove board staging code

There are no more users of the board staging code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/eec1bfb2878237888a8c3bc866d18dc53900739f.1705932585.git.geert+renesas@glider.be


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>