Clean up subnet identities storage

[cuteolaf]

Description

Clean up storage items related to subnet identities.

Related Issue(s)

• Closes #2211

Checklist

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

[JohnReedV]

Only removing the storage item definitions will leave the data on the chain. The data needs to be removed in a migration.

[cuteolaf]

Only removing the storage item definitions will leave the data on the chain. The data needs to be removed in a migration.

Done.