Remove unused Identifier() function from structs implementing atree.TypeInfo

[fxamacker]

Closes #3674

atree.TypeInfo interface used to include Identifier() function. However, Identifier() was removed later and is no longer part of atree.TypeInfo interface.

This PR removes Identifier() function from structs implementing atree.TypeInfo interface.

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• [x] Targeted PR against master branch
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [ ] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[github-actions[bot]]

Cadence Benchstat comparison

This branch with compared with the base branch onflow:master commit a56e52127ee2ff1036c8d6995ff4bc4ea7446874 The command for i in {1..N}; do go test ./... -run=XXX -bench=. -benchmem -shuffle=on; done was used. Bench tests were run a total of 7 times on each branch.

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