The API is_PSA_non_secure_target() uses obsolete labels to detect if a
target is PSA non-secure target and is not needed anymore.

Mbed OS depends on TF-M for PSA SPM and services. TF-M is built using
it's own build system. Therefore, we don't need to differentiate secure
and non-secure targets anymore in Mbed OS as all PSA targets in Mbed OS
are non-secure targets.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>