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.

include/amiconf.h

@@ -180,8 +180,6 @@ typedef unsigned short AMII_COLOR_TYPE;
 #define ANSI_DEFAULT
 #endif
 
-extern int amibbs; /* BBS mode? */
-
 #ifdef AMII_GRAPHICS
 extern int amii_numcolors;
 void amii_setpens(int);