ARM: rmobile: rcar-common: Zap arch_preboot_os()

ARM: rmobile: rcar-common: Zap arch_preboot_os()

v2018.01 commit e23eb942ad103f ("ARM: rmobile: Stop using
rcar-common/common.c on Gen3") removed
board/renesas/rcar-common/common.c from the build chain with the
reasoning that calling arch_preboot_os() is no longer needed.

However, it left the arch_preboot_os() in place. Get rid of it.
This is done in preparation of resurrecting rcar-common/common.c.

NOTE: The three removed header includes (io.h, sys_proto.h, rcar-mstp.h)
are in direct relationship with the dropped arch_preboot_os() hook. The
other headers (common.h, rmobile.h) are going to be needed by pretty
much anything that is going to appear in the rcar common code. So, keep
the two in place.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>