tfm: Remove TFM_BUILD_NS and update TFM_USE_NS_APP for NS build folder

TF-M no longer builds the NS app, but exports build files to api_ns
folder and expects the user to build the rest themselves.
Remove the option to build the NS app, and update the TFM_USE_NS_APP
to look for an output hex file in the tfm_ns folder.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
Signed-off-by: Markus Swarowsky <markus.swarowsky@nordicsemi.no>