boards: arm: lpcxpresso55s28: Remove NS board config

There is no known TFM/secure side solution for the lpcxpresso55s28.
The '_ns' board config variant was added as a copy/pasted based on
how some other boards did things.  If/when TFM supports the board
we can revert this to support it.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>