Remove CPU profiler from legacy trace viewer #5539
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The legacy trace viewer pulls a CPU profile when an event from the UI thread is selected. This profile would be shown at the bottom. The profiler has minimal samples for an event like this and doesn't allow for showing samples that spanned multiple events / flutter frames.
The legacy trace viewer is off by default (replaced with perfetto) and only accessible by enabling the setting 'Use legacy trace viewer'. At some point, we will likely remove the legacy trace viewer entirely (#5466), but for now, this PR removes the profiler from the legacy view to reduce the maintenance burden and to simplify where we need to do things like set the cpu sampling rate in initialization. When we switch to the protozero format, the perfetto view will have cpu samples included in the data (#4755).
This will also reduce the cost of having a higher sampling rate by default when a user opens the performance page. We will now only increase the sampling rate when a user opens the CPU profiler page. In the future, once the perfetto view supports showing CPU samples in the timeline, we will add a setting to the Performance page that allows a user to increase the CPU sampling rate if they choose to, but not by default. CC @dnfield - addresses part of your concerns.