fix: remove ambiguous strings4 into example

[leno23]

Summary

• remove the ambiguous "nice weather".into() prompt from strings4
• remove the corresponding solution explanation and #[allow(clippy::useless_conversion)] example
• keep strings4 focused on values that have one clear String vs &str answer before conversion traits are introduced

Fixes #2190.

Test Plan

• cargo fmt --check
• cargo check --manifest-path dev/Cargo.toml --bin strings4_sol
• cargo clippy --manifest-path dev/Cargo.toml --bin strings4_sol -- -D warnings
• cargo test --manifest-path dev/Cargo.toml --bin strings4_sol
• cargo test