[flycheck] Maybe remove :enabled check

Currently the logic looks for the `phpstan.neon` file to enable the checker.

I store this file inside `app/` or `tests/` directories with a bit different configurations for each scenario and thus the check would never trigger. I'm not sure if it's worth to have this check as phpstan also works without config and people often don't have any, especially on smaller projects.

What I do now is I pass the config via the `-c` option dynamically (I set this as buffer-local-variable in my php init hook). I have a pull request in the making that will add this option to flycheck as well.

WDYT?