filter/Chain: eliminate, just use a chain of TwoFilters instead

The ChainFilter class is extremely complicated code, and will grow to
be even more complicated when the Filter interface gets extended.
Let's just remove it; we can easily chain many TwoFilters instead.

src/filter/LoadChain.cxx

@@ -21,27 +21,29 @@
 #include "Factory.hxx"
 #include "Prepared.hxx"
 #include "plugins/AutoConvertFilterPlugin.hxx"
-#include "plugins/ChainFilterPlugin.hxx"
+#include "plugins/TwoFilters.hxx"
 #include "util/IterableSplitString.hxx"
 
 #include <string>
 
 static void
-filter_chain_append_new(PreparedFilter &chain, FilterFactory &factory,
+filter_chain_append_new(std::unique_ptr<PreparedFilter> &chain,
+			FilterFactory &factory,
 			std::string_view template_name)
 {
 	/* using the AutoConvert filter just in case the specified
 	   filter plugin does not support the exact input format */
 
-	filter_chain_append(chain, template_name,
-			    /* unfortunately, MakeFilter() wants a
-			       null-terminated string, so we need to
-			       copy it here */
-			    autoconvert_filter_new(factory.MakeFilter(std::string(template_name).c_str())));
+	chain = ChainFilters(std::move(chain),
+			     /* unfortunately, MakeFilter() wants a
+				null-terminated string, so we need to
+				copy it here */
+			     autoconvert_filter_new(factory.MakeFilter(std::string(template_name).c_str())),
+			     template_name);
 }
 
 void
-filter_chain_parse(PreparedFilter &chain,
+filter_chain_parse(std::unique_ptr<PreparedFilter> &chain,
 		   FilterFactory &factory,
 		   const char *spec)
 {