Add the migration to remove deprecated map values for root subnet

[gztensor]

Description

The added migration is for cleaning up root subnet state. The epoch is no more executed for root and state variables such as Incentive, Rank, etc. are no longer relevant for root network.

Related Issue(s)

• Closes Clear root subnet epoch values #939

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): State cleanup

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

[shamil-gadelshin]

Superseded by #1658