fix(smart-contracts): remove balances from PairInfo & fix minor errors

[nseguias]

Description and Motivation

This PR removes balances from PairInfo, as it doesn't provide any value. The balances are being held in assets already.

The previous branch, release/v2_contracts, had a couple of errors that were shown at compile time that are now fixed.

It also fixes the pool-manager tests that were not passing.

Related Issues

307

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Checklist:

• [x] I have read Migaloo's contribution guidelines.
• [x] My pull request has a sound title and description (not something vague like Update index.md)
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation.
• [x] The code is formatted properly cargo fmt --all --.
• [x] Clippy doesn't report any issues cargo clippy -- -D warnings.
• [x] I have regenerated the schemas if needed cargo schema.

[codecov[bot]]

Codecov Report

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

Project coverage is 88.50%. Comparing base (b3cde00) to head (f1fdf5b). Report is 3 commits behind head on release/v2_contracts.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/v2_contracts #321 +/- ## ======================================================== - Coverage 88.50% 88.50% -0.01% ======================================================== Files 252 252 Lines 23604 23602 -2 ======================================================== - Hits 20890 20888 -2 Misses 2714 2714 ```

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