boards: mps2_an521: remove dependencies for BUILD_WITH_TFM

We do not need to have dependencies any more on !OPENAMP
for BUILD_WITH_TFM in the mps2 an521 target, because we
now have different targets for the non-secure version of
the board and the remote core.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>