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/traceback.c b/Python/traceback.c
index 8aaee12..f88ba1d 100644
--- a/Python/traceback.c
+++ b/Python/traceback.c
@@ -801,7 +801,7 @@
         PUTS(fd, "Stack (most recent call first):\n");
     }
 
-    frame = _PyThreadState_GetFrame(tstate);
+    frame = tstate->frame;
     if (frame == NULL) {
         PUTS(fd, "<no Python frame>\n");
         return;