COMP: Remove orphaned zero-byte marker for IOScanco ISQ (exceeds GitHub 100MB limit)

The IOScanco C0004255.ISQ blob is 106,958,336 bytes, which exceeds
GitHub's 100 MB hard limit. Pushing the content-addressed blob to
this gh-pages branch produced only a zero-byte marker file at
CID/bafkreigcun2qy5mcnsyhpwjaspkds5wmanibtc2v5xwnnajgl3v2x62drm
(originally added in PR #50); the actual content was never stored.

ITK's IOScanco module reverts its content-link back to the
SHA512 form, which ExternalData resolves via the data.kitware.com
mirror (full blob available, HTTP 200, Content-Length 106958336).
The orphan marker here is removed.

Author: hjmjohnson