Remove BIO's ex_data.

No wrappers were ever added and codesearch confirms no one ever added to it
manually. Probably anyone doing complex things with BIOs just made a custom
BIO_METHOD. We can put it back with proper functions if the need ever arises.

Change-Id: Icb5da7ceeb8f1da6d08f4a8854d53dfa75827d9c
Reviewed-on: https://boringssl-review.googlesource.com/4373
Reviewed-by: Adam Langley <agl@google.com>