bpo-39946: Remove _PyThreadState_GetFrame (GH-19094)

Remove _PyRuntime.getframe hook and remove _PyThreadState_GetFrame
macro which was an alias to _PyRuntime.getframe. They were only
exposed by the internal C API. Remove also PyThreadFrameGetter type.
diff --git a/Python/pystate.c b/Python/pystate.c
index eea666b..6331a85 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -546,13 +546,6 @@
     return interp->dict;
 }
 
-/* Default implementation for _PyThreadState_GetFrame */
-static struct _frame *
-threadstate_getframe(PyThreadState *self)
-{
-    return self->frame;
-}
-
 static PyThreadState *
 new_threadstate(PyInterpreterState *interp, int init)
 {
@@ -562,10 +555,6 @@
         return NULL;
     }
 
-    if (_PyThreadState_GetFrame == NULL) {
-        _PyThreadState_GetFrame = threadstate_getframe;
-    }
-
     tstate->interp = interp;
 
     tstate->frame = NULL;
@@ -1000,9 +989,6 @@
 PyThreadState_GetInterpreter(PyThreadState *tstate)
 {
     assert(tstate != NULL);
-    if (tstate == NULL) {
-        return NULL;
-    }
     return tstate->interp;
 }
 
@@ -1011,7 +997,7 @@
 PyThreadState_GetFrame(PyThreadState *tstate)
 {
     assert(tstate != NULL);
-    return _PyThreadState_GetFrame(tstate);
+    return tstate->frame;
 }