tests: Remove use of CONFIG_ZTEST_NEW_API

The test was enabling CONFIG_ZTEST_NEW_API which
was recently removed.

(cherry picked from commit 15cbe0f)

Original-Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>
GitOrigin-RevId: 15cbe0f
Change-Id: I404d11a2cce89f4b0250fdc6e9c5e9ca6276543c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5030190
Reviewed-by: Keith Short <keithshort@chromium.org>
Commit-Queue: Keith Short <keithshort@chromium.org>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: Keith Short <keithshort@chromium.org>

tests/drivers/uart/uart_async_rx/prj.conf

@@ -1,4 +1,3 @@
 CONFIG_ZTEST=y
-CONFIG_ZTEST_NEW_API=y
 CONFIG_ZTRESS=y
 CONFIG_UART_ASYNC_RX_HELPER=y