Take thread.h out of the documentation.

There's nothing in there that isn't deprecated, since BoringSSL is thread-safe
by default now.

Change-Id: Idfd9de8bd3a6544b1d4176b2d115eef9eefa63d0
Reviewed-on: https://boringssl-review.googlesource.com/6031
Reviewed-by: Adam Langley <agl@google.com>
1 file changed