She can't just unknight him. This is not the 21st century. Furthermore a queen without a country can not really grant knighthoods to start with and she certainly can't take imginary ones away. Furthermore the knighthood is not exactly part of the nobility. Knighthoods are bestowed upon, even upon nobles, and are lost after death (they do not pass on to your heirs). Knighthood also does not come with a grant of land like it used to back when feudalism was invented. That means you can't raise a levy, you do not pay tax, you are not called upon to be a councillor which in turn means... well you are not nobility. Getting an estate in Toussaint and the appropriate title to go with it: Count or Viscount would actually make Geralt a nobleman but since he can not sire any children his line would go extinct after him.