pandad: simplify check_all_connected

[deanlee]

Simplify check_all_connected by using std::all_of and removing the do_exit update, as it's now unnecessary after the removal of the panda safety thread. The check_all_connected function is now always used with do_exit like this:

if (!do_exit && check_all_connected()) {
...
}

[github-actions]

Thanks for contributing to openpilot! In order for us to review your PR as quickly as possible, check the following:

• Convert your PR to a draft unless it's ready to review
• Read the contributing docs
• Before marking as "ready for review", ensure:
  • the goal is clearly stated in the description
  • all the tests are passing
  • the change is something we merge
  • include a route or your device' dongle ID if relevant

[adeebshihadeh]

We generally prefer to stick to C-style stuff, even at the expense of lines.