libressl: OCSP and intermediate certs workaround no longer needed

lib/vtls/openssl.c has a workaround for a bug with OCSP responses signed
by intermediate certs, this was fixed in LibreSSL in
https://github.com/libressl-portable/openbsd/commit/912c64f68f7ac4f225b7d1fdc8fbd43168912ba0

Bug: https://curl.haxx.se/mail/lib-2017-06/0038.html