Remove unnecessary cast in DynamicTextureAtlasBuilder

[mgi388]

Summary

• I started experimenting if TextureAtlas and friends can be moved to bevy_image. See Discord thread.
• While doing that, and moving DynamicTextureAtlasBuilder to bevy_image, it revealed that DynamicTextureAtlasBuilder depends on bevy_render::GpuImage, but we can't have bevy_image depend on bevy_render.
• The reason for the dependency is an assertion introduced in this PR.
• It doesn't seem like there was a specific reason for that change, so should be safe to change it.
• So instead of the cast, just look up asset_usage directly on the concrete Image type.
• Also update the message which referred to a non-existent variable atlas_texture_handle (it was renamed during a subsequent refactor PR).

Testing

• Checked on Discord if there was any known reason this had to stay like this.
• CI builds it.

[kristoff3r]

Seems reasonable