Use target_abi = "sim" instead of ios_simulator feature

[madsmtm]

Objective

Get rid of a redundant Cargo feature flag.

Solution

Use the built-in target_abi = "sim" instead of a custom Cargo feature flag, which is set for the iOS (and visionOS and tvOS) simulator. This has been stable since Rust 1.78.

In the future, some of this may become redundant if Wgpu implements proper supper for the iOS Simulator:
gfx-rs/wgpu#7057

CC @mockersf who implemented the original fix.

Testing

• Open mobile example in Xcode.
• Launch the simulator.
• See that no errors are emitted.
• Remove the code cfg-guarded behind target_abi = "sim".
• See that an error now happens.

(I haven't actually performed these steps on the latest main, because I'm hitting an unrelated error (EDIT: It was #17637). But tested it on 0.15.0).

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Migration Guide

If you're using a project that builds upon the mobile example, remove the ios_simulator feature from your Cargo.toml (Bevy now handles this internally).