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 1bb01b1b5f27a7de33e7a67946b8c001b54e09e9.

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