Extracted `Piwik::getJavascriptCode()` into a separate, non static class: `Piwik\Tracker\TrackerCodeGenerator`

`Piwik::getJavascriptCode()` is now deprecated in favor of the new class.
Removed `Piwik::getJavascriptCode()` and its deprecation notice: it wasn't an API method
Merge pull request #6446 from piwik/refactoring-tracker-code-generator

Extracted `Piwik::getJavascriptCode()` into a separate, non static class