Revert "enable TasksMax= for all services by default, and set it to 512"

Introducing a default limit on number of threads broke a lot of software which
regularly needs more, such as MySQL and RabbitMQ, or services that spawn off an
indefinite number of subtasks that are not in a scope, like LXC or cron.

512 is way too much for most "simple" services, and it's way too little for the
ones mentioned above. Effective (and much stricter) limits should instead be
put into units individually.

Closes: #823530
LP: #1578080