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 b55ff319f880adc874b8c95957adf2003117d42b.

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