avfilter/Makefile: remove dnn files when calling make clean

Also remove the ancient reference to libmpcodecs while at it.
Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>

libavfilter/Makefile

@@ -526,7 +526,7 @@ TESTPROGS = drawutils filtfmts formats integral
 TOOLS-$(CONFIG_LIBZMQ) += zmqsend
 
 clean::
-	$(RM) $(CLEANSUFFIXES:%=libavfilter/libmpcodecs/%)
+	$(RM) $(CLEANSUFFIXES:%=libavfilter/dnn/%)
 
 OPENCL = $(subst $(SRC_PATH)/,,$(wildcard $(SRC_PATH)/libavfilter/opencl/*.cl))
 .SECONDARY: $(OPENCL:.cl=.c)