Update Unused using linting rule to not report warning by default

[timotheeguerin]

A few things to investigate

1. It shouldn't report a warning by default(just gray out in the IDE)
2. When enabled it should also report in the CLI

The result here is that this doesn't get fixed and create noise for anyone opening specs

[RodgeFu]

1. To just gray out in the IDE needs to mark these diagnostics as Hint level. I remember we talked about this and because we are not having hint level diagnostic in typespec compiler, so have to report them as warning instead.
2. These warnings are reported as linter rules and will be reported if the linter rules are enabled. Just IDE will enable these rules by default but not in CLI. do you mean we should also enable them in CLI by default?

btw, do you know why people find them noisy but not just fix them which is easy?

[timotheeguerin]

I can say for other people but I find them noisy, every spec I open in the spec repo now has immediate warning showing in the problem window which makes it harder to look at the other problems when trying to fix everything.

The problem with hint is not what we report to Vscode but introducing it in the compiler diagnostics. The lsp could have some extra code that run those unused logic