tests: drivers: syscon: remove dev pointer check

`DEVICE_DT_GET()` guarantees that the `dev` pointer can never
be `NULL`, the check is therefore redundant, remove it.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

Author: ycsin

tests/drivers/syscon/src/main.c

@@ -18,7 +18,6 @@ ZTEST(syscon, test_size)
 	const size_t expected_size = DT_REG_SIZE(DT_NODELABEL(syscon));
 	size_t size;
 
-	zassert_not_null(dev);
 	zassert_ok(syscon_get_size(dev, &size));
 	zassert_equal(size, expected_size, "size(%zu) != expected_size(%zu)", size,
 		      expected_size);