utils/misc/vim: added target in Makefile to remove old ser vim files

- make clean-ser

Makefile

@@ -5,3 +5,8 @@ install:
 	mkdir -p ~/.vim/syntax
 	cp ftdetect/kamailio.vim ~/.vim/ftdetect/
 	cp syntax/kamailio.vim ~/.vim/syntax/
+
+# remove old files using 'ser' name
+clean-ser:
+	rm -f ~/.vim/ftdetect/ser.vim
+	rm -f ~/.vim/syntax/ser.vim