ScheduleTree: drop isl::ctx-based constructor

This public constructor is not in use anymore.  Furthermore,
constructing a ScheduleTree with no concrete type is dangerous and
should not be possible.

Author: ftynse

tc/core/polyhedral/schedule_tree.cc

@@ -152,8 +152,6 @@ static std::unique_ptr<ScheduleTree> makeElem(const ScheduleTree& st) {
 ////////////////////////////////////////////////////////////////////////////////
 //                        ScheduleTree
 ////////////////////////////////////////////////////////////////////////////////
-ScheduleTree::ScheduleTree(isl::ctx ctx) : ctx_(ctx) {}
-
 ScheduleTree::~ScheduleTree() {}
 
 ScheduleTree::ScheduleTree(const ScheduleTree& st)

tc/core/polyhedral/schedule_tree.h

@@ -154,7 +154,6 @@ struct ScheduleTree {
   ScheduleTree(const ScheduleTree& st);
 
  public:
-  explicit ScheduleTree(isl::ctx ctx);
   virtual ~ScheduleTree();
 
   bool operator==(const ScheduleTree& other) const;