RISC-V: Remove __vdso_cmpxchg{32,64} symbol versions

These were left over from an earlier version of the port.
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/vdso/vdso.lds.S

@@ -70,8 +70,6 @@ VERSION
 	LINUX_4.15 {
 	global:
 		__vdso_rt_sigreturn;
-		__vdso_cmpxchg32;
-		__vdso_cmpxchg64;
 	local: *;
 	};
 }