[ Next breaking change ] Rip out unused hot restart bootstrap from `package:build_web_compilers`

[grouma]

We migrated the bootstrap logic to package:dwds in a non-breaking fashion. The next breaking change should remove all of the unused cruft in:
https://github.com/dart-lang/build/blob/master/build_web_compilers/lib/src/dev_compiler_bootstrap.dart

See dart-lang/webdev#898 for details