libast: cleanup: remove unused usage.h header

This heaer was added in 2000-06-01 93j, but nothing has ever
included it, even in the entire history of the AT&T AST repo.

src/lib/libast/Mamfile

@@ -4821,7 +4821,7 @@ make install virtual
 	note * #include headers
 	make ${INSTALLROOT}/lib/mam/ast
 		note * ...main AST headers
-		loop HDR ast ast_dir ast_std ast_windows ccode cdt cmdarg debug dt error find ftwalk fts glob hash hashkey hashpart ip6 ls magic mc mime mnt modecanon modex namval option proc recfmt regex sfio sfio_s sfio_t sfdisc shcmd stk stak swap tar times tm tok usage vdb vmalloc wait magicid fnv aso
+		loop HDR ast ast_dir ast_std ast_windows ccode cdt cmdarg debug dt error find ftwalk fts glob hash hashkey hashpart ip6 ls magic mc mime mnt modecanon modex namval option proc recfmt regex sfio sfio_s sfio_t sfdisc shcmd stk stak swap tar times tm tok vdb vmalloc wait magicid fnv aso
 			make ${INCLUDE_AST}/${HDR}.h
 				prev include/${HDR}.h
 				exec - cp -f ${<} ${@}