[flutter_tools] Remove `--null-assertions` flag for web builds

[nshahan]

When the support for the --no-sound-null-safety flag gets completely removed we should also remove --null-assertions because it only has meaning when running without sound null safety. It should get cleaned up for both the dev and production code paths in the flutter CLI tools.

See: dart-lang/sdk#50711

cc @christopherfujino

[christopherfujino]

Thanks for filing this. I'm going to close this and update instead #118810 to include this (I'm working on a PR for this already).

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