fix: Remove unsused sellCollateral functionallity

[AntonAndell]

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• [ ] I have performed a self-review of my own code
• [ ] I have documented my code in accordance with the documentation guidelines
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.13%. Comparing base (5dc3c8a) to head (99e4690). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #457 +/- ## ========================================= Coverage 73.13% 73.13% Complexity 1592 1592 ========================================= Files 115 115 Lines 7637 7637 Branches 930 930 ========================================= Hits 5585 5585 Misses 1652 1652 Partials 400 400 ``` | [Flag](https://app.codecov.io/gh/balancednetwork/balanced-java-contracts/pull/457/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=balancednetwork) | Coverage Δ | | |---|---|---| | [java](https://app.codecov.io/gh/balancednetwork/balanced-java-contracts/pull/457/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=balancednetwork) | `73.13% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=balancednetwork#carryforward-flags-in-the-pull-request-comment) to find out more.

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