* 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.