Removed connectivity monitor.

[matux]

Description of the change

I'm removing ConnectivityMonitor for the following reasons:

• Flutter already has a connectivity monitoring mechanism.
• ConnectivityMonitor doesn't provide actual connectivity monitoring in neither rollbar_common nor rollbar_dart.
• Our CM doesn't add any features over Flutter's existent implementation.
• It's not used internally.
• Less code to maintain.
• YAGNI

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

• Fix Remove ConnectivityMonitor #41

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[matux]

@akornich If you disagree, or I'm missing something, or would like to keep it, please do tell me!

[akornich]

LGTM, assuming there will be a functional replacement to the ConnectivityMonitor