refactor: drop redundant continue and remove needless_continue allow

[MsfPablo]

Drops three needless continue expressions caught by clippy::needless_continue and removes the lint's allow entry from the project lint allowlist, advancing issue #4071.

Changes:

• src/proto/h1/dispatch.rs: end of else branch in dispatch loop
• src/proto/h2/upgrade.rs: empty-buffer arm in poll_read loop
• src/proto/h2/client.rs: end of Poll::Ready(Some(...)) arm in dispatch loop
• Cargo.toml: drop needless_continue = "allow"

Verification:

• cargo clippy --features full -- -W clippy::needless_continue is clean
• cargo build --features full succeeds
• cargo test --features full --lib passes (115 passed, 0 failed)

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