Delete unreachable default clauses.

[stereotype441]

The Dart analyzer will soon be changed so that if the default clause of a switch statement is determined to be unreachable by the exhaustiveness checker, a new warning of type
unreachable_switch_default will be issued. This parallels the behavior of the existing unreachable_switch_case warning, which is issued whenever a case clause of a switch statement is determined to be unreachable. For details see
dart-lang/sdk#54575.

This PR deletes unreachable default clauses from devtools now, to avoid a spurious warning when the analyzer change lands.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or there is a reason for not adding tests.

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