s390: io: remove dummy relaxed accessor macros for reads

These are now defined by asm-generic/io.h, so we don't need the private
definitions anymore.

Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>