Special:BookSources: Remove link to PriceSCAN

This price comparison service seems to have shut down in 2011. The
ISBN link likely has been broken for years, and there is no working
URL on the site itself to replace it with.

Change-Id: Ib3e61bbd86302f885bff75cb92fa87d35d657d5f
Merge "Special:BookSources: Remove link to PriceSCAN"