chore(tls): remove redundant i/o bounds

[cratelyn]

this commit removes a redundant set of trait bounds from linkerd_tls::Client<L, C>'s tower::Service<T> implementation.

this client type is generic over a C-typed MakeConnection. this trait is effectively an alias for particular services, and already by definition is prerequisite upon Connection responses that are an asynchronous reader/writer.

see the definition of the trait, here:

// linkerd/stack/src/connect.rs

pub trait MakeConnection<T> {
    /// An I/O type that represents a connection to the remote endpoint.
    type Connection: AsyncRead + AsyncWrite;

    /// Metadata associated with the established connection.
    type Metadata;

    type Error: Into<Error>;

    type Future: Future<Output = Result<(Self::Connection, Self::Metadata), Self::Error>>;

    /// Determines whether the connector is ready to establish a connection.
    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>>;

    /// Establishes a connection.
    fn connect(&mut self, t: T) -> Self::Future;

    // contd...
}

thus, we can remove these bounds from the tls client. the connection is already, by virtue of C: MakeConnection, an AsyncRead + AsyncWrite type.

see linkerd/linkerd2#8733.