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:

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

@@                   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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[0xFable]

LGTM but remove file build_rs_cov.profraw unless we need it