Makefile: Use $(RM), silencing errors on missing files

$(RM) includes the -f flag, so the clean target now succeeds when files
to remove don't exist. The post-condition of clean is that compilation
artifacts are not present; this is trivially satisfied if they never
existed.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>

Makefile

@@ -27,6 +27,6 @@ example4: example4.o genann.o
 
 
 clean:
-	rm *.o
+	$(RM) *.o
-	rm *.exe
+	$(RM) *.exe
-	rm persist.txt
+	$(RM) persist.txt