cmake: remove obsolete CONFIG_ZTEST_NEW_API from CMakeLists.txt

Follow-up: #49243, #48732

With the introduction of Kconfig for unit_testing board and setting
ZTEST_NEW_API in testcase.yaml, the need and support of setting this
directly in CMakeLists.txt is obsolete.

Remove the code line.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>