Resolve "Remove ` index_users_with_static_object_token`"

What does this MR do and why?

We have introduced the index index_users_with_static_object_tokento help the performance of the EncryptStaticObjectToken background migration. This background migration was introduced in 2021.

Since we have already finished this background migrations, we don't need to have this index anymore.

Merge request where we introduced the index: !76684 (merged)

Index statement:

CREATE INDEX index_users_with_static_object_token ON users USING btree (id) WHERE ((static_object_token IS NOT NULL) AND (static_object_token_encrypted IS NULL));

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Related to #429091 (closed)