This removes FakeTicker and FakeTickerProvider from devtools's tests as they are unused.

This change does not require release notes since it only affects devtools's tests.

Motivation

FakeTicker and FakeTickerProvider implement Ticker and TickerProvider respectively. Since the devtools repo is in Flutter's customer tests, this effectively blocks us from adding any new members to Ticker and TickerProvider.

For example, flutter/flutter#173862 adds a new member to Ticker. This caused customer test failures: link

| Analyzing devtools_app...
|
|   error - test/test_infra/utils/rendering_tester.dart:377:7 - Missing concrete implementations of 'getter class Ticker.forceFrames' and 'setter class Ticker.forceFrames'. Try implementing the missing methods, or make the class abstract. - non_abstract_class_inherits_abstract_member

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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