Do not fail if _matrix-identity DNS SRV record missing

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/963

This also simplifies Prerequisites, which is great.

It'd be nice if we were doing these checks in some optional manner
and reporting them as helpful messages (using
`matrix_playbook_runtime_results`), but that's more complicated.
I'd rather drop these checks completely.

docs/prerequisites.md

@@ -20,8 +20,6 @@ If your distro runs within an [LXC container](https://linuxcontainers.org/), you
 
 - The [Ansible](http://ansible.com/) program being installed on your own computer. It's used to run this playbook and configures your server for you. Take a look at [our guide about Ansible](ansible.md) for more information, as well as [version requirements](ansible.md#supported-ansible-versions) and alternative ways to run Ansible.
 
-- Either the `dig` tool or `python-dns` installed on your own computer. Used later on, by the playbook's [services check](maintenance-checking-services.md) feature.
-
 - An HTTPS-capable web server at the base domain name (`<your-domain>`) which is capable of serving static files. Unless you decide to [Serve the base domain from the Matrix server](configuring-playbook-base-domain-serving.md) or alternatively, to use DNS SRV records for [Server Delegation](howto-server-delegation.md).
 
 - Properly configured DNS records for `<your-domain>` (details in [Configuring DNS](configuring-dns.md)).