Refactor: scheduler: don't include originally unpacked resource parameters in digest

Previously, when calculating an operation digest, calculate_main_digest() would
grab the following, in order of highest to lowest precedence:

• instance attributes evaluated for the appropriate node
• instance attributes specified with the operation
• instance attributes as originally unpacked (without evaluating for any node)
• resource meta-attributes

Adding the originally unpacked instance attributes was redundant, since
node-evaluated instance attributes would always be a superset of those and
would take precedence.