Revert "kernel-doc: Use c:struct for Sphinx 3.0 and later"

This reverts commit 152d1967.
We will replace the commit with the fix from Linux.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201117165312.118257-16-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>