Remove unused section from Cargo.toml

[rparrett]

Fixes this warning seen when running any example

warning: unused manifest key: target.wasm32-unknown-unknown.runner

As noted here: https://github.com/jakobhellermann/wasm-server-runner#step-2 this only works in an individual's cargo config.

[bzm3r]

Big if true! Has this been tested with: https://github.com/StarArawn/bevy_ecs_tilemap#running-examples-on-web

[rparrett]

Those instructions do continue to work. The removed section is completely superfluous / doing nothing for us.

However, wasm-server-runner really shines when configured as described in the link above. If configured properly, the second command isn't necessary at all. Cargo will use the configured runner automatically and all you have to do is (note: run instead of build)

cargo run --target wasm32-unknown-unknown --example animation --release --features atlas

I added a commit that improves the instructions.