Remove differentiation code for pre v0.7 (#203)

Author: blegat

src/differentiation.jl

@@ -87,23 +87,10 @@ differentiate(p::AbstractArray, x::Union{AbstractArray, Tuple}, deg::Int) = _dif
 differentiate(p, x, ::Val{0}) = p
 differentiate(p, x, ::Val{1}) = differentiate(p, x)
 
-@static if VERSION < v"v0.7.0-"
-    # Marking this @pure helps julia v0.6 figure this out
-    Base.@pure _reduce_degree(::Val{N}) where {N} = Val{N - 1}()
-    function differentiate(p, x, deg::Val{N}) where N
-        if N < 0
-            throw(DomainError(deg))
-        else
-            differentiate(differentiate(p, x), x, _reduce_degree(deg))
-        end
-    end
-else
-    # In Julia v0.7 and above, we can remove the _reduce_degree trick
-    function differentiate(p, x, deg::Val{N}) where N
-        if N < 0
-            throw(DomainError(deg))
-        else
-            differentiate(differentiate(p, x), x, Val{N - 1}())
-        end
+function differentiate(p, x, deg::Val{N}) where N
+    if N < 0
+        throw(DomainError(deg))
+    else
+        differentiate(differentiate(p, x), x, Val{N - 1}())
     end
 end