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: NJan Vesely <jano.vesely@gmail.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

po/Makefile

@@ -32,7 +32,7 @@ update: $(SRCS)
 build: $(OBJS)
 
 clean:
-	$(RM) $(OBJS)
+	rm -f $(OBJS)
 
 install: $(OBJS)
 	for obj in $(OBJS); do \