build: simplify libavfilter configure checks

This is all not needed anymore. In particular, remove all configure
switches except --enable-libavfilter.

waftools/fragments/libavfilter.c

@@ -1,7 +0,0 @@
-#include <libavfilter/avfilter.h>
-static void vf_next_query_format() {}
-int main(void) {
-    avfilter_register_all();
-    vf_next_query_format();
-    return 0;
-}