s390: delete the unused store_prefix() function

Its last usage was deleted in commit 4df29d2 ("s390/smp: rework
absolute lowcore access").

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: iii-i

arch/s390/include/asm/lowcore.h

@@ -222,12 +222,4 @@ static inline void set_prefix(__u32 address)
 	asm volatile("spx %0" : : "Q" (address) : "memory");
 }
 
-static inline __u32 store_prefix(void)
-{
-	__u32 address;
-
-	asm volatile("stpx %0" : "=Q" (address));
-	return address;
-}
-
 #endif /* _ASM_S390_LOWCORE_H */