Reverts flutter/flutter#143517

Initiated by: dnfield

Reason for reverting: broke CI, see https://ci.chromium.org/ui/p/flutter/builders/prod/Linux%20web_benchmarks_skwasm/3446/overview

Original PR Author: mkustermann

Reviewed By: {eyebrowsoffire}

This change reverts the following previous change:
Original Description:
* Make `flutter build web` have one option that determins the optimization level: `-O<level>` / `--optimization-level=<level>` => Defaulting to -O4 => Will apply to both dart2js and dart2wasm

* Deprecate `--dart2js-optimization=O<level>`

* Disentagle concept of optimization from concept of static symbols => Add a `--strip-wasm` / `--no-strip-wasm` flag that determins whether static symbols are kept in the resulting wasm file.

* Remove copy&past'ed code in the tests for wasm build tests

* Cleanup some artifacts code, now that we no longer use `wasm-opt` inside flutter tools