serial: sh: Drop assorted SH3, SH4, SH5 macros

Drop unused macros from the header to clean it up.

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>