Revert "PrefixSearch (ApiOpenSearch) now supports searching in multiple namespaces"

Caused regression in opensearch results for the main search field on every
page. Special pages are no longer included when they should. 

This reverts commit 0e054bbf1a955b02afd4feaa04063c841e19a569.

Change-Id: Ib39c6d7816cd3c21ec714f5d6272bebd049ac430
Merge "Revert "PrefixSearch (ApiOpenSearch) now supports searching in multiple namespaces""