The "marking" are not really significant. I do not think you will find a statute which talking about unauthorized removing of "markings".
The documents matter. The contents of the documents, that is. The State Dept had documents which contained classified information. Therefore, they were marked. Hillary wanted those documents to be in an unauthorized location (big NO-NO according to statute). In order to facilitate getting the classified documents into an unauthorized location, Hillary instructed her employee to remove markings. This is bad.
It's not the removal of the markings. It's the re-location of the documents.
Her intent was CLEARLY in violation of statute.
Except the “smoking gun” document referred to a non functional secure fax, so the issue wasn’t the location but expediency of transfer to a presumably allowed location via improperly insecure means.