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 738911cde68b2b3706e502cf8daf5b14738f2f42.

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

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit 03af843039af758fc9bbb4ae6c09ec2bc715f2c5)