fix: crystal next compatibility

refs https://github.com/crystal-lang/crystal/pull/11597
fixes https://github.com/crystal-ameba/ameba/issues/406

spec/ameba/ast/variabling/assignment_spec.cr

@@ -85,30 +85,5 @@ module Ameba::AST
         assignment.branch.should be_nil
       end
     end
-
-    describe "#transformed?" do
-      it "returns false if the assignment is not transformed by the compiler" do
-        nodes = as_nodes <<-CRYSTAL
-          def method(a)
-            a = 2
-          end
-          CRYSTAL
-        scope = Scope.new nodes.def_nodes.first
-        variable = Variable.new(nodes.var_nodes.first, scope)
-        assignment = Assignment.new(nodes.assign_nodes.first, variable, scope)
-        assignment.transformed?.should be_false
-      end
-
-      it "returns true if the assignment is transformed by the compiler" do
-        nodes = as_nodes <<-CRYSTAL
-          array.each do |(a, b)|
-          end
-          CRYSTAL
-        scope = Scope.new nodes.block_nodes.first
-        variable = Variable.new(nodes.var_nodes.first, scope)
-        assignment = Assignment.new(nodes.assign_nodes.first, variable, scope)
-        assignment.transformed?.should be_true
-      end
-    end
   end
 end

spec/ameba/rule/lint/useless_assign_spec.cr

@@ -233,7 +233,7 @@ module Ameba::Rule::Lint
       subject.catch(s).should be_valid
     end
 
-    context "when transformed" do
+    context "block unpacking" do
       it "does not report if the first arg is transformed and not used" do
         s = Source.new %(
           collection.each do |(a, b)|

src/ameba/ast/variabling/assignment.cr

@@ -76,6 +76,9 @@ module Ameba::AST
       end
     end
 
+    # TODO: Remove in a next release. BC for crystal <= 1.9.
+    # refs https://github.com/crystal-ameba/ameba/pull/407
+    #
     # Indicates whether the node is a transformed assignment by the compiler.
     # i.e.
     #