lavfi: remove 'opaque' parameter from AVFilter.init()

It is not used in any filters currently and is inherently evil. If
passing binary data to filters is required in the future, it should be
done with some AVOptions-based system.

libavfilter/avfilter.h

@@ -456,10 +456,8 @@ typedef struct AVFilter {
     /**
      * Filter initialization function. Args contains the user-supplied
      * parameters. FIXME: maybe an AVOption-based system would be better?
-     * opaque is data provided by the code requesting creation of the filter,
-     * and is used to pass data to the filter.
      */
-    int (*init)(AVFilterContext *ctx, const char *args, void *opaque);
+    int (*init)(AVFilterContext *ctx, const char *args);
 
     /**
      * Filter uninitialization function. Should deallocate any memory held