Revert "RT3425: constant-time evp_enc"

Causes more problems than it fixes: even though error codes
are not part of the stable API, several users rely on the
specific error code, and the change breaks them. Conversely,
we don't have any concrete use-cases for constant-time behaviour here.

This reverts commit 4aac102f75b517bdb56b1bcfd0a856052d559f6e.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Add a comment noting the padding oracle.

Reviewed-by: Andy Polyakov <appro@openssl.org>