[prim,fpv] Remove initialisation before a test

[rswarbrick]

This initialisation isn't really needed because the array will come up as 'x and the SboxPermutationCheck_A assertion below would fail if the end result wasn't '1.

The existing code also causes a reported coverage hole in the FPV flow, which notes that assigning sbox_perm_test to zero has no effect.

Obviously, this is a little silly: it's inside ifndef SYNTHESIS so coverage doesn't really matter! But waiving this sort of coverage hole more generally might be tricky and this particular case can just be "fixed".