remove unnecessary error return

I was running "golangci-lint" and found this two warnings

internal/checker/checker.go:135:18: (*Checker).LoadIndex$3 - result 0 (error) is always nil (unparam)
        final := func() error {
                        ^
internal/repository/repository.go:457:18: (*Repository).LoadIndex$3 - result 0 (error) is always nil (unparam)
        final := func() error {
                        ^

It turns out that these functions are used only in "RunWorkers(...)",
which is used only two times in whole project right after this "final"
functions.
And because these "final" functions always return "nil", I've
descided, that it would be better to remove requriments for "final" func
to return error to avoid magick "return nil" at their end.

internal/checker/checker.go

@@ -132,9 +132,8 @@ func (c *Checker) LoadIndex(ctx context.Context) (hints []error, errs []error) {
 	}
 
 	// final closes indexCh after all workers have terminated
-	final := func() error {
+	final := func() {
 		close(resultCh)
-		return nil
 	}
 
 	// run workers on ch