Remove a host of unnecessary OPENSSL_IS_BORINGSSL ifdefs

Remove a host of unnecessary OPENSSL_IS_BORINGSSL ifdefs

The <openssl/buf.h> include is relevant in both OpenSSL and BoringSSL
because the file uses BUF_MEM (include what you use). OpenSSL just
happened to include it via another file. OpenSSL also spells it
<openssl/buffer.h>, not matching the type, so use the compatible
spelling.

Additionally all the CHECKED_CAST and manual STACK_OF(T) definitions
call into BoringSSL internals. The correct, public APIs are simply to
just use the same code as OpenSSL and call the DEFINE_STACK_OF macros.

Signed-off-by: David Benjamin <davidben@google.com>