This commit series removes the following associated
function from the Violation trait:
fn placeholder() -> Self;
ruff previously used this placeholder approach for the messages it
listed in the README and displayed when invoked with --explain <code>.
This approach is suboptimal for three reasons:
1. The placeholder implementations are completely boring code since they
just initialize the struct with some dummy values.
2. Displaying concrete error messages with arbitrary interpolated values
can be confusing for the user since they might not recognize that the
values are interpolated.
3. Some violations have varying format strings depending on the
violation which could not be documented with the previous approach
(while we could have changed the signature to return Vec<Self> this
would still very much suffer from the previous two points).
We therefore drop Violation::placeholder in favor of a new macro-based
approach, explained in commit 4/5.
Violation::placeholder is only invoked via Rule::kind, so we firstly
have to get rid of all Rule::kind invocations ... this commit starts
removing the trivial cases.
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