Mark SSL3_ENC_METHODs const and remove an unused one.

There's an undefined one not used anywhere. The others ought to be const.  Also
move the forward declaration to ssl.h so we don't have to use the struct name.

Change-Id: I76684cf65255535c677ec19154cac74317c289ba
Reviewed-on: https://boringssl-review.googlesource.com/2561
Reviewed-by: Adam Langley <agl@google.com>
6 files changed