Clean up subnet identities storage

[cuteolaf]

Description

Clean up storage items related to subnet identities.

Related Issue(s)

• Closes Clean up subnet identities unused storage #2211

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 ./scripts/fix_rust.sh 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

[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.