Remove unused main() function in day-2.morning exercise.

[gendx]

This exercise is about making the unit tests pass, so we can remove the main() function, which is empty and unused. Otherwise the Rust playground complains about dead code.

[gendx]

Looks like the cargo build check is not happy about that.

[mgeisler]

Looks like the cargo build check is not happy about that.

Yeah, it's probably listed as a binary in the Cargo.toml file and so the build breaks without a main.

[mgeisler]

We could unlist it there if it's covered equivalently by mdbook test.

[mgeisler]

This looks a bit stale — we can revisit it after #1073 is merged.