Straighten out remote.List{WithContext}

[imjasonh]

ListWithContext is kinda gross when we also have List(..., remote.WithContext(ctx) -- let's deprecate it.

edit: the rest is no longer relevant, but useful for context:
This led a test to fail because the /v2/ request fails on a cancelled context, which I think is good because we shouldn't ignore the provided context for that ping request. In fixing that, I wanted to make the test check for whether the error errors.Is(context.Canceled), but that was stymied by transport.NewTransportWithContext losing that information when it does it's little multi-error thing.

    list_test.go:115: error was not context.Canceled, got Get "https://127.0.0.1:60957/v2/": context canceled; Get "http://127.0.0.1:60957/v2/": context canceled

Gross.

So we have a couple options:

1. change the test to just search the error message for the string "context canceled" and keep the http fallback multierror logic from before (probably safest)
2. don't return multiple errors, maybe if logs.Enabled(logs.Debug) { logs.Debug.Printf("failed https, falling back to http") }
3. drop this whole thing, ListWithContext is fine the way it is, and ignoring context to do the ping is fine.