Remove stake entries with zero stake

[JohnReedV]

Description

This PR:

• Adds unstaking to remove_network
• Adds migration to remove all stake entries with 0 stake.
• Adds unit tests for remove_network and remove_zero_stakes_v1 migration

Related Issue(s)

• Closes #[issue number]

Type of Change

• [x] 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):