Refactorings in ScheduledReports to remove useless indentation

I have refactored the code a bit to make it (hopefully) more readable. I have replaced this:

```php
if (self::manageEvent($reportType)) {
    // do something
}
```

By this:

```php
if (! self::manageEvent($reportType)) {
    return;
}

// do something
```

It's not much but it removes one level of indentation in almost the whole class, which is kind of nice.

FYI, this is one of the "object calisthenics" rules.
Merge pull request #6396 from piwik/code-refactorings

Refactoring in ScheduledReports to remove useless indentation