Remove unused nghttp2_mem_chunk

lib/includes/nghttp2/nghttp2.h

@@ -688,24 +688,6 @@ typedef struct {
   int32_t window_size_increment;
 } nghttp2_window_update;
 
-/**
- * @struct
- *
- * The structure to hold chunk of memory.
- *
- * TODO Drop this if it is not used anymore.
- */
-typedef struct {
-  /**
-   * The pointer to the data.
-   */
-  uint8_t *data;
-  /**
-   * The length of the data.
-   */
-  size_t length;
-} nghttp2_mem_chunk;
-
 /**
  * @union
  *