Fix comment for $wgUseImageResize

It is not obsolete - it is used by the Bitmap class. I tested it and it
does indeed disable server-side image thumbnailing. The variable was
originally un-obsoleted in r48787, but accidentally re-marked obsolete
when merging upload-api/js2 branch in r53282.

Also moving it to a more logical location in DefaultSettings.

Change-Id: Ibb4d9784e997ea3b6ca7e2e0272635e6c8a30d26
Merge "Fix comment for $wgUseImageResize"