Remove unnecessary code causing troubles on Debian-based systems

It should be `/bin/mkdir` and `/bin/chown` on Ubuntu 18.04 for example.
Still, it doesn't seem like we need to create and chown these
directories at all, since the playbook takes care of creating them
and setting appropriate permission by itself.

roles/matrix-server/templates/systemd/matrix-postgres.service.j2

@@ -7,8 +7,6 @@ Requires=docker.service
 Type=simple
 ExecStartPre=-/usr/bin/docker stop matrix-postgres
 ExecStartPre=-/usr/bin/docker rm matrix-postgres
-ExecStartPre=-/usr/bin/mkdir {{ matrix_postgres_data_path }}
-ExecStartPre=-/usr/bin/chown {{ matrix_user_uid }}:{{ matrix_user_gid }} {{ matrix_postgres_data_path }}
 ExecStart=/usr/bin/docker run --rm --name matrix-postgres \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \