Cleanup: Remove obsolete code in interface_panel.c

Some code delt with panel merging in earlier versions of Blender,
which is no longer needed. Other code delt with controls that aren't
used anymore, and in some cases have region-level equivalents.

There's a surprising amount of this unused code in this file, so removing it
will be helpful for the future.

Differential Revision: https://developer.blender.org/D7938

source/blender/makesdna/DNA_screen_types.h

@@ -167,9 +167,8 @@ typedef struct Panel {
   /** Panel size excluding children. */
   int blocksizex, blocksizey;
   short labelofs;
-  char _pad[2];
+  char _pad[4];
   short flag, runtime_flag;
-  short control;
   short snap;
   /** Panels are aligned according to increasing sort-order. */
   int sortorder;
@@ -539,8 +538,8 @@ enum {
   PNL_CLOSEDX = (1 << 1),
   PNL_CLOSEDY = (1 << 2),
   PNL_CLOSED = (PNL_CLOSEDX | PNL_CLOSEDY),
-  /*PNL_TABBED    = (1 << 3), */ /*UNUSED*/
-  PNL_OVERLAP = (1 << 4),
+  /* PNL_TABBED = (1 << 3), */  /*UNUSED*/
+  /* PNL_OVERLAP = (1 << 4), */ /*UNUSED*/
   PNL_PIN = (1 << 5),
   PNL_POPOVER = (1 << 6),
   /** The panel has been drag-drop reordered and the instanced panel list needs to be rebuilt. */