Trailing commas are swift-format's default now that oida holds no rule

[3lvis]

Why

multiElementCollectionTrailingCommas: false was a truce, and the other side of it is gone.

swift-format defaults the option to true, so it adds a trailing comma to a multi-line literal. oida's trailing_comma rule defaulted to mandatoryComma = false, so it flagged that same comma. Two default-on tools with opposite instructions on one character — the key is what kept them from undoing each other on every run. oida 0.16.0 deleted trailing_comma ("Trailing commas are the formatter's"), so the key now referees a fight with one side in it.

With it gone, swift-format's default stands: a multi-line literal ends with a comma, so adding an element touches one line rather than two and the diff says what changed. Sixteen lines picked that up from bin/lint --fix.

.swift-format is down to the three keys that say something — a version, a width, an indent.

The approach

Measured the same way as #332: formatting the tree with and without the key, and reading what moved. The products: array keeps its shape, since the option speaks only for collections with several elements.

Testing

make test (180 tests, green) plus bin/lint --check clean.

Learnings

A config key that duplicates a default is easy to spot; one that contradicts a default is the one worth re-reading, because it usually holds a truce with some other tool, and truces outlive their counterparties silently. #332 pruned the duplicates and kept this one on the grounds that it changed output — which answered whether it did something, rather than whether the something was still wanted.