crypto: drop gcrypt thread initialization code

This is only required on gcrypt < 1.6.0, and is thus obsolete
since

  commit b33a84632a3759c00320fd80923aa963c11207fc
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Fri May 14 13:04:08 2021 +0100

    crypto: bump min gcrypt to 1.8.0, dropping RHEL-7 support

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>