selftests: devmem: remove unused variable

Trivial fix to unused variable.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250615203511.591438-2-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/hw/ncdevmem.c

@@ -852,7 +852,6 @@ static int do_client(struct memory_buffer *mem)
 	ssize_t line_size = 0;
 	struct cmsghdr *cmsg;
 	char *line = NULL;
-	unsigned long mid;
 	size_t len = 0;
 	int socket_fd;
 	__u32 ddmabuf;