Cleanup page indexer request class

[markusguenther]

• adjusting some php docs
• remove an unused variable $pageId in the getHeader method. It was also duplicated because itemId will be equal to the pageId in the page indexer
• Prevent warnings while indexing pages. The pages will be logged with errors so why should i see the warning in the scheduler?

[irnnr]

Please don't use the @ silencer

[markusguenther]

But in this case you will get warnings in the backend and this is also not so nice ... you will see the error later in the backend module or devlog.