Remove Legacy REST documentation

[ryanchristo]

Summary

Following #9594, we will need to remove/update legacy REST documentation. This should be done after #9567 has been merged and backported to release/v0.43.x. Documentation that will need to be removed/updated includes the following:

• https://docs.cosmos.network/master/basics/app-anatomy.html
• https://docs.cosmos.network/master/core/grpc_rest.html
• https://docs.cosmos.network/master/migrations/rest.html

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