* Remove usages of StackTraceMapper

This class will be replaced with a function type in `test_api`, clean up
usages ahead of time.

In `build_script.dart` the only reference to `StackTraceMapper` is in
the `browser_test.dart` file, but since nothing would call
`setStackTraceMapper` from that import it should be safe to remove the
definition. After reading a serialized mapper from the
`test.browser.mapper` channel, it was ignored.

In `flutter_web_platform.dart` we only were reading from `_mappers`,
never putting values in, so we can remove it entirely. The `load()`
function was never called with a `mapper` argument.

This reverts commit 3aa7a800.

This reverts commit 868b0e6d.

This reverts commit c2353b29.

This reverts commit 9d4a35ba because it appears to break

the gradle_non_android_plugin_test.

Now that the new schema is supported on the stable channel, and the old
schema is considered legacy, the template should always create plugins
using the new schema.

The stocks example app is outdated and deprecated, but we still use it for some benchmark tests. Moving it into the benchmarks directory to indicate its status.