Revert "Add abc_box_id attribute to MUXF7/F8 cells"

This reverts commit 8fbbd9b129.

techlibs/xilinx/cells_sim.v

@@ -159,12 +159,10 @@ module MUXCY(output O, input CI, DI, S);
   assign O = S ? CI : DI;
 endmodule
 
-(* abc_box_id = 1 *)
 module MUXF7(output O, input I0, I1, S);
   assign O = S ? I1 : I0;
 endmodule
 
-(* abc_box_id = 2 *)
 module MUXF8(output O, input I0, I1, S);
   assign O = S ? I1 : I0;
 endmodule