bevy_asset: Add missing web-sys feature and cleanup unused ones

[snendev]

Objective

• Describe the objective or issue this PR addresses.

bevy_asset includes code here that references web_sys::WorkerGlobalScope. However, bevy_asset does not enable this feature, see here. Running examples does not catch this problem because the feature is implicitly included by wgpu when bevy_render is also a dependency, see bevy_render and wgpu. This results in compile errors for environments that are not using bevy_render.

To reproduce the problem, try to build the crate individually for wasm targets by running cargo build -p bevy_asset --target wasm32-unknown-unknown.

Running cargo tree -e features --target wasm32-unknown-unknown helped diagnose the issue.

Solution

• Describe the solution used to achieve the objective above.

This PR adds the WorkerGlobalScope feature to the web-sys portion of bevy_asset's Cargo.toml.

It also seems to be the case that bevy_asset no longer needs the Request feature, since no code for Request is present anymore. I confirmed that building the crate individually for wasm succeeds without the feature, so that change is also included here.

This is a little off-topic, but the repository would probably benefit from some automation around these types of changes, but I'm not sure what would work there. For example, building each crate individually for some key targets would work, but is...well, a lot. Happy to follow up if there is agreement on a good direction.

Testing

• Did you test these changes? If so, how?
• How can other people (reviewers) test your changes? Is there anything specific they need to know?

Building the crate individually for wasm by running cargo build -p bevy_asset --target wasm32-unknown-unknown.

• Are there any parts that need more testing?

I don't believe so.

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