Remove unnecessary guard in TreeMap

This guard was added in fb09e82 but does
not seem to be necessary. Note that `UntypedTreeMap` handles the case
`DerivingTemplate` explicitly.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1595,7 +1595,8 @@ object Trees {
               cpy.DefDef(tree)(name, transformParamss(paramss), transform(tpt), transform(tree.rhs))
             case tree @ TypeDef(name, rhs) =>
               cpy.TypeDef(tree)(name, transform(rhs))
-            case tree @ Template(constr, parents, self, _) if tree.derived.isEmpty =>
+            case tree @ Template(constr, parents, self, _) =>
+              assert(tree.derived.isEmpty, s"Expected `derived` to be empty in: " + tree)
               cpy.Template(tree)(transformSub(constr), transform(tree.parents), Nil, transformSub(self), transformStats(tree.body, tree.symbol))
             case Import(expr, selectors) =>
               cpy.Import(tree)(transform(expr), selectors)