Don't purge thumbs for old versions of an image during ?action=purge

This tends to become extremely expensive as the number of
oldversions of a file increase. It is also not generally
needed since ?action=purge is usually targeting the
current version (additionally old versions of the file
have fixed urls, so they're less likely to get out
of sync). If an old version does need to be purged, one
can revdel and un-revdel it.

It should be noted that this extra purging was added
for bug 30192. However, since that bug was fixed,
most of the places requiring purging of old thumbnails
have now done it directly instead of by relying on
$file->purgeCache. The exception being revision
deletion, which still assumes $file->purgeCache clears
the thumbnails of old versions of the file.

There's no real bug for this, but I kind of hijacked
bug 49362

Change-Id: Ib399132cabe79fd2b4b23bad5708bfa50b282074
Merge "Don't purge thumbs for old versions of an image during ?action=purge"