[libpng16] Reverted addition of png_set_filler_16 and png_set_add_alpha_16()

functions. They unnecessarily duplicate png_set_filler() and png_set_add_alpha()
which now work properly with 16-bit images.
diff --git a/pngrtran.c b/pngrtran.c
index 208f4ce..284226a 100644
--- a/pngrtran.c
+++ b/pngrtran.c
@@ -2660,17 +2660,10 @@
    png_uint_32 i;
    png_uint_32 row_width = row_info->width;
 
-   png_byte lo_filler = (png_byte)filler;
-
 #ifdef PNG_READ_16BIT_SUPPORTED
    png_byte hi_filler = (png_byte)(filler>>8);
-   if (((flags & PNG_FLAG_FILLER_16) == 0) && row_info->bit_depth == 16)
-   {
-      /* filler bytes were read in the wrong order prior to libpng-1.5.24 */
-      lo_filler = hi_filler;
-      hi_filler = (png_byte)filler;
-   }
 #endif
+   png_byte lo_filler = (png_byte)filler;
 
    png_debug(1, "in png_do_read_filler");