Remove incorrect characteristic(R::ResidueField)

Author: fingolfin

src/ResidueField.jl

@@ -64,11 +64,6 @@ function modulus(r::ResFieldElem)
    return modulus(parent(r))
 end
 
-characteristic(R::ResidueField) = characteristic(base_ring(R))
-# FIXME: why is the above method correct in general??? Isn't it wrong if
-# we e.g. start with ZZ[:x] and factor out ideal([x, 2]) ?
-is_known(::typeof(characteristic), R::ResidueField) = is_known(characteristic, base_ring(R))
-
 characteristic(r::ResidueField{T}) where T <: Integer = modulus(r)
 is_known(::typeof(characteristic), R::ResidueField{T}) where T <: Integer = true