[1.24wmf6] Revert "Use square bounding boxes for default-sized thumbnails"

Reverting for further discussion, due to community complaints at
https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_%28technical%29&oldid=611154541#Infobox_Image

This reverts commit 7db55341791cb44ca599b84bea707b05de2f227d.

Change-Id: I6234e58912ac05aa6f58b16a32f8ec7eb3aeaec9