The unused ex_data index declarations are commented out instead of
removed so that it is clear which values to avoid for any new ex_data
indexes added in the future.

Change-Id: Ia19da9631324492c5c7eeacc71453e6240c73870
Reviewed-on: https://boringssl-review.googlesource.com/3940
Reviewed-by: Adam Langley <agl@google.com>