Improve 2020/yang/try.sh

The make clobber now removes decoded, code.c and encoded.c which try.sh
creates.

2020/yang/Makefile

@@ -208,7 +208,7 @@ clean:
 
 clobber: clean
 	${RM} -f ${TARGET} ${ALT_TARGET}
-	${RM} -rf *.dSYM
+	${RM} -rf *.dSYM decoded code.c encoded.c
 	@-if [ -e sandwich ]; then \
 	    ${RM} -f sandwich; \
 	    echo 'ate sandwich'; \