RISC-V: Clean up an unused include

We used to have some cmpxchg syscalls.  They're no longer there, so we
no longer need the include.

CC: Christoph Hellwig <hch@infradead.org>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/sys_riscv.c

@@ -14,7 +14,6 @@
  */
 
 #include <linux/syscalls.h>
-#include <asm/cmpxchg.h>
 #include <asm/unistd.h>
 #include <asm/cacheflush.h>