docs: Avoid warning on cleandocs

Recent Makefile changes added an rm command without the requisite "-f",
leading to warnings if the files do not exist.  Make it be quiet again.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/media/Makefile b/Documentation/media/Makefile
index 297b85c..c22a30b 100644
--- a/Documentation/media/Makefile
+++ b/Documentation/media/Makefile
@@ -38,7 +38,7 @@
 latex: $(IMGPDF) all
 
 clean:
-	-rm $(IMGTGT) 2>/dev/null
+	-rm -f $(IMGTGT) 2>/dev/null
 
 $(BUILDDIR):
 	$(Q)mkdir -p $@
@@ -85,4 +85,4 @@
 	@$($(quiet)gen_rst)
 
 cleandocs:
-	-rm ${TARGETS}
+	-rm -f ${TARGETS}