bool_comparison: no longer lint on !x != y

[ada4a]

The issue that this PR was opened to fix (#15367), talked about how, for !x == false, this lint suggests both:

1. x != false
2. x

I initially wanted to make the lint short-circuit after emitting the first suggestion (since that was the easier change code-wise), but that was not optimal for two reasons:

1. suggestion 2 is obviously better (the other one requires one more round of Clippy)
2. suggestion 1 is actually already covered by nonminimal_bool

And so I instead decided to completely remove the code path that emits suggestion 1.

Fixes #15367

changelog: [bool_comparison]: no longer lint on !x != y

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