Revert "alter default disk chunk sizes"

This reverts commit 3068e55e51.

libs/ardour/diskstream.cc

@@ -777,20 +777,12 @@ Diskstream::default_disk_read_chunk_frames()
 #elif defined __APPLE__
 	return (4 * 1048576) / sizeof (Sample);
 #else
-	/* Linux, etc. */
-	return (2 * 1048576) / sizeof (Sample);
+	return 65536;
 #endif
 }	
 
 framecnt_t
 Diskstream::default_disk_write_chunk_frames ()
 {
-#ifdef PLATFORM_WINDOWS
-	return (2 * 1048576);
-#elif defined __APPLE__
-	return (4 * 1048576);
-#else
-	/* Linux, etc. */
 	return 65536;
-#endif
 }