Finally, we can remove this JS global function for customizing the url strategy.

Why I think we don't need to go through an official deprecation process:

1. It was initially made for internal use in Google3, and right now there are no references to it.
2. There's no public documentation of this JS function.
3. External users customize their url strategy through `flutter_web_plugins` which has been [migrated](flutter/flutter#123443) already.

…... (#42463)

If this roll has caused a breakage, revert this CL and stop the roller
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https://dart.googlesource.com/sdk.git/+log/e0caea366989..78d09b5874fa

2023-05-31 [email protected] Version 3.1.0-159.0.dev
2023-05-31 [email protected] Version 3.1.0-158.0.dev
2023-05-31 [email protected] Version 3.1.0-157.0.dev
2023-05-31 [email protected] Version 3.1.0-156.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
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Reverts #42134

This is blocking the engine into framework roller:

See: https://cirrus-ci.com/task/5610586755563520

```
Analyzing 3 items...                                            
  error � The class 'UrlStrategy' can't be extended outside of its library because it's an interface class � dev/integration_tests/web_e2e_tests/test_driver/url_strategy_integration.dart:48:31 � invalid_use_of_type_outside_library
1 issue found. (ran in 321.8s)
  �  �  
  ```