Remove broken DSA private key workarounds.

Remove old code that handled various invalid DSA formats in ancient
software.

This also fixes a double free bug when parsing malformed DSA private keys.

Thanks to Adam Langley (Google/BoringSSL) for discovering this bug using
libFuzzer.

CVE-2016-0705

Reviewed-by: Emilia Käsper <emilia@openssl.org>