chore: enable linters.exclusions.warn-unused

[alexandear]

We can remove some exclusions:

$ golangci-lint run
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "test/testdata_etc", Linters: "gofmt, goimports"] 
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "internal/x", Linters: "gofmt, goimports"] 
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "pkg/commands/internal/migrate/two/", Linters: "lll"] 
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "internal/go", Linters: "gofmt, goimports"] 
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "pkg/goformatters/gci/internal", Linters: "gofmt, goimports"] 
WARN [runner/exclusion_rules] Skipped 0 issues by rules: [Path: "pkg/goanalysis/runner_checker.go", Linters: "gofmt, goimports"] 
0 issues.

[ldez]

I didn't enable this option because it has side effects on the tests.

Also, the current exclusions should be kept because this is related to borrowed code.