Cleanup: Remove unused ChunkType flag CH_AUTO_LENGTH

CH_AUTO_LENGTH is no longer used anywhere, so remove all code
that depends on it.

src/saveload/saveload.cpp

@@ -1753,32 +1753,6 @@ static void SlLoadCheckChunk(const ChunkHandler *ch)
 	}
 }
 
-/**
- * Stub Chunk handlers to only calculate length and do nothing else.
- * The intended chunk handler that should be called.
- */
-static ChunkSaveLoadProc *_stub_save_proc;
-
-/**
- * Stub Chunk handlers to only calculate length and do nothing else.
- * Actually call the intended chunk handler.
- * @param arg ignored parameter.
- */
-static inline void SlStubSaveProc2(void *arg)
-{
-	_stub_save_proc();
-}
-
-/**
- * Stub Chunk handlers to only calculate length and do nothing else.
- * Call SlAutoLenth with our stub save proc that will eventually
- * call the intended chunk handler.
- */
-static void SlStubSaveProc()
-{
-	SlAutolength(SlStubSaveProc2, nullptr);
-}
-
 /**
  * Save a chunk of data (eg. vehicles, stations, etc.). Each chunk is
  * prefixed by an ID identifying it, followed by data, and terminator where appropriate
@@ -1794,12 +1768,6 @@ static void SlSaveChunk(const ChunkHandler *ch)
 	SlWriteUint32(ch->id);
 	DEBUG(sl, 2, "Saving chunk %c%c%c%c", ch->id >> 24, ch->id >> 16, ch->id >> 8, ch->id);
 
-	if (ch->flags & CH_AUTO_LENGTH) {
-		/* Need to calculate the length. Solve that by calling SlAutoLength in the save_proc. */
-		_stub_save_proc = proc;
-		proc = SlStubSaveProc;
-	}
-
 	_sl.block_mode = ch->flags & CH_TYPE_MASK;
 	switch (ch->flags & CH_TYPE_MASK) {
 		case CH_RIFF:

src/saveload/saveload.h

@@ -409,7 +409,6 @@ enum ChunkType {
 	CH_SPARSE_ARRAY =  2,
 	CH_TYPE_MASK    =  3,
 	CH_LAST         =  8, ///< Last chunk in this array.
-	CH_AUTO_LENGTH  = 16,
 };
 
 /**