fix: remove not necessary zero variable

[pandadefi]

Description

Prior to vyper 0.3.4, 0 defaulted to uint256 which was forcing us to create a zero variable to compare.

Fixes # (issue)

Checklist

• [ ] I have run vyper and solidity linting
• [ ] I have run the tests on my machine
• [ ] I have followed commitlint guidelines
• [ ] I have rebased my changes to the latest version of the main branch

[storming0x]

Prior to vyper 0.4.4,

do you mean 0.3.6?