chore: remove unused code

[moflotas]

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[dkharms]

There is a comment for DialContext method:

// One subtle difference between NewClient and Dial and DialContext is that the
// former uses "dns" as the default name resolver, while the latter use
// "passthrough" for backward compatibility.  This distinction should not matter
// to most users, but could matter to legacy users that specify a custom dialer
// and expect it to receive the target string directly.

I believe these is no difference for us, but could please you double-check it? (if you have not done this already, of course)