fix: wrong return type for drop transaction

[PierreFlowdesk]

Pull Request Description

The return type of the dropTransaction method is wrong. https://developers.fireblocks.com/reference/droptransaction

Type of change

Please delete options that are not relevant.

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Chore / Documentation
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• [X] Locally tested against Fireblocks API

Checklist:

• [X] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] Any dependent changes have been merged and published in downstream modules
• [ ] I have added corresponding labels to the PR