Stop requesting coverage reports for web apps

[kenzieschmoll]

When connected to a web app, we get an error from the getSourceReport RPC:

getSourceReport: (-32602) Source report kind Coverage not supported

We should not be requesting coverage for a web app. See https://github.com/flutter/devtools/blob/master/packages/devtools_app/lib/src/screens/debugger/codeview_controller.dart/#L284.

[kenzieschmoll]

This exception is caught and logged as a warning by a try / catch.