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/editors/include/UI_interface.h

@@ -175,13 +175,6 @@ enum {
   UI_RETURN_POPUP_OK = 1 << 5,
 };
 
-/* panel controls */
-enum {
-  UI_PNL_SOLID = 1 << 1,
-  UI_PNL_CLOSE = 1 << 5,
-  UI_PNL_SCALE = 1 << 9,
-};
-
 /* but->flag - general state flags. */
 enum {
   /** Warning, the first 6 flags are internal. */