Fix the error reporting on old-style API

Unfortunately, the way we reported the use of an old-style API also triggered
when not using it. Change that to only trigger when we DO use the old API, by
marking the `combo_t` constructor private.

Unfortunately, this does not allow us to use a custom error message.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/Kaleidoscope/MagicCombo.h

@@ -31,13 +31,6 @@
   }                                                                       \
   }
 
-#define _MAGICCOMBO_API_CHANGE                                            \
-  "The MagicCombo API changed in an incompatible way, you will need to\n" \
-  "upgrade.\n"                                                            \
-  "\n"                                                                    \
-  "Please see the `UPGRADING.md` document shipped with the source:\n"     \
-  " https://github.com/keyboardio/Kaleidoscope-MagicCombo/blob/master/UPGRADING.md"
-
 namespace kaleidoscope {
 
 class MagicCombo : public kaleidoscope::Plugin {
@@ -50,10 +43,8 @@ class MagicCombo : public kaleidoscope::Plugin {
   typedef struct combo_t {
     uint32_t left_hand, right_hand;
 
-    combo_t& operator=(combo_t &) {
-      static_assert(false, _DEPRECATE(_MAGICCOMBO_API_CHANGE));
-      return *this;
-    }
+   private:
+    combo_t(byte left, byte right) {}
   } combo_t;
 
   MagicCombo(void) {}