Amiga: delete dead files and dead-macro code

Delete amigst.c (empty) and amitty.c (TTY/BBS stub).  Drop WINVERS_AMIT,
SUPERBITMAP_MAP, EXTMENU, SHELL/dosh(), and the bbs_id reference in
src/files.c.

sys/amiga/winext.h

@@ -59,9 +59,6 @@ extern struct IOStdReq ConsoleIO;
 extern struct MsgPort *HackPort;
 
 extern int txwidth, txheight, txbaseline;
-#ifdef SUPERBITMAP_MAP
-extern struct BitMap amii_vbm;
-#endif
 
 /* This gadget data is replicated for menu/text windows... */
 extern struct PropInfo PropScroll;