sh: Remove SH5-based Cayman platform

Since the removal of core support for SH5, Cayman support can no longer
be selected.

Fixes: 37744fee

 ("sh: remove sh5 support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rich Felker <dalias@libc.org>