This change also drops ex_data from DH objects. The global would need
special handling in the FIPS module, which isn't hard, but just dropping
it saves some of the code-size costs of this change and I cannot find
any signs of use of this functionality.
Change-Id: I984bd70698c2ec329f340d294b3b9ec169cd0c4e
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44524
Reviewed-by: David Benjamin <davidben@google.com>