Move Dash logger.setLevel to init + remove now redundant startup message

[anders-kiaer]

Resolves #1354.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Move setLevel to __init__
  • Remove two now redundant messages.
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

Optionals

• I have added entry in the CHANGELOG.md (not added, this PR probably is not regarded as a "notable change", let me know if an entry should be added).
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follow
  • this github #PR number updates the dash docs
  • here is the show and tell thread in plotly dash community

[alexcjohnson]

Nice! Actually I do think this warrants a changelog entry since it enables users to do something new. Something like:

Removed redundant log message and consolidated logger initialization. You can now control the log level - for example suppress informational messages from Dash with app.logger.setLevel(logging.WARNING).

(I don't think it's even necessary to say when you can use setLevel, since it's anytime after you have an app object and before you run the server)

[anders-kiaer]

👍 Agree - new changelog entry added 📜 🔖 Not sure if it belongs as an "Added" entry or a "Fixed" entry (it adds new configuration possibility with setLevel, but does that mainly through a small fix).

[alexcjohnson]

Excellent - thanks! 💃