[2.7] bpo-29738: Fix memory leak in _get_crl_dp (GH-526) (GH-1144)

* Remove conditional on free of `dps`, since `dps` is now allocated for
all versions of OpenSSL
* Remove call to `x509_check_ca` since it was only used to cache
the `crldp` field of the certificate
CRL_DIST_POINTS_free is available in all supported versions of OpenSSL
(recent 0.9.8+) and LibreSSL.
(cherry picked from commit 2849cc34a8db93d448a62d69c462402347b50dcb)
1 file changed