clippy: use strip_prefix instead of manually strip

        let libressl_prefix = "RUST_VERSION_LIBRESSL_";

        let boringsl_prefix = "RUST_OPENSSL_IS_BORINGSSL";

        let conf_prefix = "RUST_CONF_";

        if line.starts_with(openssl_prefix) {

            let version = &line[openssl_prefix.len()..];

        if let Some(version) = line.strip_prefix(openssl_prefix) {

            openssl_version = Some(parse_version(version));

        } else if line.starts_with(new_openssl_prefix) {

            let version = &line[new_openssl_prefix.len()..];

        } else if let Some(version) = line.strip_prefix(new_openssl_prefix) {

            openssl_version = Some(parse_new_version(version));

        } else if line.starts_with(libressl_prefix) {

            let version = &line[libressl_prefix.len()..];

        } else if let Some(version) = line.strip_prefix(libressl_prefix) {

            libressl_version = Some(parse_version(version));

        } else if line.starts_with(conf_prefix) {

            enabled.push(&line[conf_prefix.len()..]);

        } else if let Some(conf) = line.strip_prefix(conf_prefix) {

            enabled.push(conf);

        } else if line.starts_with(boringsl_prefix) {

            is_boringssl = true;

        }