tests: i2c_emul: Remove unused variable

Building with clang warns:

tests/drivers/i2c/i2c_emul/src/test_forwarding_buf.cpp:20:32: error:
unused variable 'targets' [-Werror,-Wunused-const-variable]
constexpr const struct device *targets[FORWARD_COUNT] = {
                               ^

Signed-off-by: Tom Hughes <tomhughes@chromium.org>

tests/drivers/i2c/i2c_emul/src/test_forwarding_buf.cpp

@@ -15,8 +15,6 @@ namespace
 
 /* Get the devicetree constants */
 constexpr const struct device *controller = DEVICE_DT_GET(CONTROLLER_LABEL);
-constexpr const struct device *targets[FORWARD_COUNT] = {
-	DT_FOREACH_PROP_ELEM_SEP(CONTROLLER_LABEL, forwards, DEVICE_DT_GET_BY_IDX, (,))};
 
 ZTEST(i2c_emul_forwarding, test_write_is_forwarded)
 {