Revert r23197 -- while well-meaning, this would severely overrepresent minor content...

Revert r23197 -- while well-meaning, this would severely overrepresent minor content namespaces by selecting the namespaces with equal probability.
A better solution would be to modify the RandomPage class to allow passing pages of multiple namespaces, and giving it the complete list of content namespaces; then the selection would be properly proportional.