protocols/horizon: Remove deprecated API fields

[Shaptic]

Epic: #5433

---

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:

go/protocols/horizon/main.go

Lines 180 to 187 in dfabe31

	// Action needed in release: horizon-v3.0.0: deprecated field
	NumAccounts int32 `json:"num_accounts"`
	NumClaimableBalances int32 `json:"num_claimable_balances"`
	NumLiquidityPools int32 `json:"num_liquidity_pools"`
	NumContracts int32 `json:"num_contracts"`
	NumArchivedContracts int32 `json:"num_archived_contracts"`
	// Action needed in release: horizon-v3.0.0: deprecated field
	Amount string `json:"amount"`

• 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:

go/protocols/horizon/main.go

Lines 536 to 538 in dfabe31

	// Action needed in release: horizon-v3.0.0: remove valid_(after|before)
	ValidAfter string `json:"valid_after,omitempty"`
	ValidBefore string `json:"valid_before,omitempty"`

[sreuland]

should this effort include a coordinated outreach/blast to sdk's for notice? a separate ticket for cleanup of the same in js-sdk?