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.

roles/matrix-base/tasks/main.yml

@@ -32,10 +32,3 @@
     - setup-ma1sd
     - setup-synapse
     - setup-nginx-proxy
-
-- import_tasks: "{{ role_path }}/tasks/self_check_dns.yml"
-  delegate_to: 127.0.0.1
-  become: false
-  when: run_self_check|bool
-  tags:
-    - self-check