build_helper_configs.py: Remove ns_target arg

With TF-M commit [1], users can skip specifying NS output target name in
build command line.
Remove ns_target related settings.

[1] https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/24353

Change-Id: I9d7602e5d6d6d2c068427c6e92e7feebd1271d18
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed