[Mob] Remove Silver beastcoin from THF quest quadavs

[ThrisStraizo]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

This PR removes the Silver Beastcoin from the steal list of the Quadav Stew mobs.

The stew is the only stealable item from these mobs, if the thf quest is active or not.

Steps to test these changes

Steal a stew.