[Fix] allow unstake below nom limit

[camfairchild]

Description

This reapplies the fix from #383 which was inadvertently removed in #366.
This also fixes an issue with the hotfix, removing the nomination storage entry when the Stake is sweeped.

Related Issue(s)

• Closes Allow remove stake below nom minimum #454

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

NA

Additional Notes

NA