Revert "Prerender thumbnails at upload time on all wikis except commons"

Suspected cause of spikes in 500 responses from Varnish. Filippo had said
something to this point on the original change but we let it go anyway.

This reverts commit e7f1edb1ce3064c59b10c5c6f751142732ddc0be.

Change-Id: I0ad921efb2f8f0ade05b0c91d0904006dde8f880
Merge "Revert "Prerender thumbnails at upload time on all wikis except commons""