[prim,fpv] Remove initialisation before a test

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".

Signed-off-by: Rupert Swarbrick <[email protected]>

hw/ip/prim/rtl/prim_lfsr.sv

@@ -419,7 +419,6 @@ module prim_lfsr #(
       // Check that the permutation is indeed a permutation.
       logic [LfsrDw-1:0] sbox_perm_test;
       always_comb begin : p_perm_check
-        sbox_perm_test = '0;
         for (int k = 0; k < LfsrDw; k++) begin
           sbox_perm_test[sbox_in_indices[k]] = 1'b1;
         end