Remove hardcoded constant folding for unary minus in cabs2cil

[karoliineh]

We want integer overflow safety checks from our internal dashboard implementation to be reported for unary minus operations to constants. However, when constant folding is enabled, this case should still handled in cil.ml:

| UnOp(unop, e1, tres) -> begin
  ...
  match unop with
    Neg -> const_if_not_overflow (UnOp(Neg,Const(CInt(i,ik,s)),tres)) tk (neg_cilint ic)
    ...