po/Makefile: call rm -f directly

Default variables are undefined in rules.mak and this is what the rest
of the build system uses.
Fixes make clean in ./po/

Signed-off-by: Jan Vesely <jano.vesely@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/po/Makefile b/po/Makefile
index b271f79..7bab09d 100644
--- a/po/Makefile
+++ b/po/Makefile
@@ -32,7 +32,7 @@
 build: $(OBJS)
 
 clean:
-	$(RM) $(OBJS)
+	rm -f $(OBJS)
 
 install: $(OBJS)
 	for obj in $(OBJS); do \