Remove DisableReportRange.unusedRule

[nex3]

(Added by request in https://github.com/stylelint/stylelint/pull/4897/files#r470950362)

#4897 renames the DisableReportRange.unusedRule field to .rule, since it's used to refer to rules in multiple contexts (rather than just when they're unused). We're keeping the old field name around for the time being to ensure backwards-compatibility, but in the next major version release the old name should be removed entirely.

[ota-meshi]

Note that vscode-stylelint also uses that unusedRule field.
https://github.com/stylelint/vscode-stylelint/blob/c34a0cefd5e18ddf695261eca19651eee825ae46/lib/stylelint-vscode/index.js#L434
https://github.com/stylelint/vscode-stylelint/blob/f6f367214fbfd4ad6568abecead0bed76cabda75/server.js#L455

[jeddy3]

but in the next major version release the old name should be removed entirely.

SGTM. I'll set the milestone to future-major.

[ybiquitous]

Closed via #5345