Remove superfluous methods on Grads

As far as I can tell, these were never used, tested or documented.
Moreover, they don't make sense semantically since `Grads` behaves like a Dict rather than an ordinal-indexed collection like an Array.
Meanwhile, their continued existence is causing issues like #1484.

src/compiler/interface.jl

@@ -375,40 +375,6 @@ function Base.copy(gs::Grads)
   merge!(gs_new, gs)
 end
 
-broadcasted(f, gs::Grads, gss::ADictOrGrads...) = map(f, gs, gss...)
-
-broadcasted(f, a::Numeric, gs::Grads) = map(x -> f(a, x), gs)
-broadcasted(f, gs::Grads, a::Numeric) = map(x -> f(x, a), gs)
-
-function materialize!(gs1::Grads, gs2::Grads)
-  issetequal(gs1.params, gs2.params) || 
-    throw(ArgumentError("Expected Grads objects with the same Params."))
-  for p in gs1.params
-    gs1[p] = gs2[p]
-  end
-  return gs1
-end
-
-
-function Base.map(f, gs1::Grads, gss::ADictOrGrads...)
-  gsout = Grads(IdDict{Any,Any}(), Params(gs1.params))
-  return map!(f, gsout, gs1, gss...)
-end
-
-function Base.map!(f, gsout::Grads, gss::ADictOrGrads...)
-  all(issetequal(gsout.params, keys(gs)) for gs in gss) ||
-    throw(ArgumentError("map! expects Grads objects with the same Params."))
-  for p in gsout.params
-    gsout[p] = f((_getformap(gs, p) for gs in gss)...)
-  end
-  return gsout
-end
-
-function _getformap(gs, p)
-  g = gs[p]
-  isnothing(g) ? fill!(similar(p), 0) : g
-end
-
 function pullback(f, ps::Params)
   cx = Context{true}(nothing)
   y, back = _pullback(cx, f)