As of https://boringssl-review.googlesource.com/#/c/6980/, almost all of
BoringSSL #ifdefs in cURL should be unnecessary:

- BoringSSL provides no-op stubs for compatibility which replaces most
  #ifdefs.

- DES_set_odd_parity has been in BoringSSL for nearly a year now. Remove
  the compatibility codepath.

- With a small tweak to an extend_key_56_to_64 call, the NTLM code
  builds fine.

- Switch OCSP-related #ifdefs to the more generally useful
  OPENSSL_NO_OCSP.

The only #ifdefs which remain are Curl_ossl_version and the #undefs to
work around OpenSSL and wincrypt.h name conflicts. (BoringSSL leaves
that to the consumer. The in-header workaround makes things sensitive to
include order.)

This change errs on the side of removing conditionals despite many of
the restored codepaths being no-ops. (BoringSSL generally adds no-op
compatibility stubs when possible. OPENSSL_VERSION_NUMBER #ifdefs are
bad enough!)

Closes #640