Revert "Add vp9_highbitdepth info in configure --help"

This reverts commit a6715a75589b418f9af5f44d93e0d7ecbaa77d48.

Removes a duplicate entry; this was previously added by:
14e37cf Add help info for --enable-vp9-highbitdepth

Change-Id: I61408e782232821ef6ed84775b5c79d172ba7f2d

configure

diff --git a/configure b/configure
index 80149c2..88df374 100755
--- a/configure
+++ b/configure
@@ -58,7 +58,6 @@
   ${toggle_postproc_visualizer}   macro block / block level visualizers
   ${toggle_multi_res_encoding}    enable multiple-resolution encoding
   ${toggle_temporal_denoising}    enable temporal denoising and disable the spatial denoiser
-  ${toggle_vp9_highbitdepth}      enable 10/12 bit support in VP9
   ${toggle_vp9_temporal_denoising}
                                   enable vp9 temporal denoising
   ${toggle_webm_io}               enable input from and output to WebM container