ARM: omap1: remove osk-mistral add-on board support

As Aaro Koskinen points out, nobody should have this one any more,
and I noticed the code is rather ugly, so let's removed it but
keep the rest of the OSK support that is still used.

Link: https://lore.kernel.org/linux-arm-kernel/20221020193511.GB3019@t60.musicnaut.iki.fi/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>