Remove the ability to set custom ex_data implementations.

This is never used and we can make the built-in one performant.

Change-Id: I6fc7639ba852349933789e73762bc3fa1341b2ff
Reviewed-on: https://boringssl-review.googlesource.com/4370
Reviewed-by: Adam Langley <agl@google.com>
8 files changed