RISC-V: remove spin_unlock_wait()

This was removed from the other architectures in commit
952111d7 ("arch: Remove spin_unlock_wait() arch-specific
definitions").  That landed between when we got upstream and when our
patches were reviewed, so this is a followup patch.
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/include/asm/spinlock.h

@@ -58,15 +58,6 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
 	}
 }
 
-static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
-{
-	smp_rmb();
-	do {
-		cpu_relax();
-	} while (arch_spin_is_locked(lock));
-	smp_acquire__after_ctrl_dep();
-}
-
 /***********************************************************/
 
 static inline void arch_read_lock(arch_rwlock_t *lock)