Remove unnecessary index on cluster_agent_tokens

What does this MR do and why?

Removes index index_cluster_agent_tokens_on_agent_id_and_last_used_at on cluster_agent_tokens, which is unnecessary after !77923 (merged) and !78420 (merged).

Screenshots or screen recordings

== 20220117034056 RemoveIndexClusterAgentTokensOnAgentIdAndLastUsedAt: migrating
-- transaction_open?()
   -> 0.0000s
-- indexes(:cluster_agent_tokens)
   -> 0.0129s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- remove_index(:cluster_agent_tokens, {:algorithm=>:concurrently, :name=>"index_cluster_agent_tokens_on_agent_id_and_last_used_at"})
   -> 0.0067s
-- execute("RESET statement_timeout")
   -> 0.0012s
== 20220117034056 RemoveIndexClusterAgentTokensOnAgentIdAndLastUsedAt: migrated (0.0252s)

== 20220117034056 RemoveIndexClusterAgentTokensOnAgentIdAndLastUsedAt: reverting
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:cluster_agent_tokens, "agent_id, last_used_at DESC NULLS LAST", {:name=>"index_cluster_agent_tokens_on_agent_id_and_last_used_at", :algorithm=>:concurrently})
   -> 0.0042s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:cluster_agent_tokens, "agent_id, last_used_at DESC NULLS LAST", {:name=>"index_cluster_agent_tokens_on_agent_id_and_last_used_at", :algorithm=>:concurrently})
   -> 0.0046s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20220117034056 RemoveIndexClusterAgentTokensOnAgentIdAndLastUsedAt: reverted (0.0122s)

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