protocols/horizon: Remove deprecated API fields

[Shaptic]

Epic: #5433

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There are several API fields that have been deprecated for a very long time. We should remove them as part of the Protocol 22 release in which we will change to the protocol-specific versioning scheme (i.e. horizon-v22.0.0).

The following API fields are deprecated:

• In /assets, the num_accounts and amount fields:

https://github.com/stellar/go/blob/dfabe31b5b89cb235e81732d708a3f5bdeff507a/protocols/horizon/main.go#L180-L187

• In /transactions/:id (or any paginating variant), the valid_before and valid_after fields, because users should instead check the transaction's preconditions which can have way more than time bounds:

https://github.com/stellar/go/blob/dfabe31b5b89cb235e81732d708a3f5bdeff507a/protocols/horizon/main.go#L536-L538