bpf: remove unused variable

fix the following warning
../kernel/bpf/syscall.c: In function ‘map_lookup_and_delete_elem’:
../kernel/bpf/syscall.c:1010:22: warning: unused variable ‘ptr’ [-Wunused-variable]
  void *key, *value, *ptr;
                      ^~~

Fixes: bd513cd08f ("bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/syscall.c

@@ -1007,7 +1007,7 @@ static int map_lookup_and_delete_elem(union bpf_attr *attr)
 	void __user *uvalue = u64_to_user_ptr(attr->value);
 	int ufd = attr->map_fd;
 	struct bpf_map *map;
-	void *key, *value, *ptr;
+	void *key, *value;
 	u32 value_size;
 	struct fd f;
 	int err;