sh: rsk7203: Remove the board

sh: rsk7203: Remove the board

Last change to this board was done in 2016, has no prospects of
ever being converted to DM, drop it.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Vladimir Zapolskiy <vz@mleia.com>
Cc: Yoshihiro Shimoda <shimoda.yoshihiro.uh@renesas.com>