Cleanup used membership invites

What does this MR do?

Cleanups remaining used membership invites. There was a bug which caused invite tokens being generated for users who already accepted an invite. It was fixed in: !67760 (merged) Here we cleanup remaining invalid tokens as separate data migration

Migration output

== 20210825150212 CleanupRemainingOrphanInvites: reverting ====================
-- indexes(:members)
   -> 0.0309s
-- current_schema()
   -> 0.0022s
== 20210825150212 CleanupRemainingOrphanInvites: reverted (0.0376s) ===========

== 20210825150212 CleanupRemainingOrphanInvites: migrating ====================
-- transaction_open?()
   -> 0.0001s
-- index_exists?(:members, :id, {:where=>"invite_token IS NOT NULL AND user_id IS NOT NULL", :name=>"tmp_idx_members_with_orphaned_invites", :algorithm=>:concurrently})
   -> 0.0343s
-- add_index(:members, :id, {:where=>"invite_token IS NOT NULL AND user_id IS NOT NULL", :name=>"tmp_idx_members_with_orphaned_invites", :algorithm=>:concurrently})
   -> 0.0125s
-- transaction_open?()
   -> 0.0000s
-- indexes(:members)
   -> 0.0266s
-- remove_index(:members, {:algorithm=>:concurrently, :name=>"tmp_idx_members_with_orphaned_invites"})
   -> 0.0034s
== 20210825150212 CleanupRemainingOrphanInvites: migrated (0.0993s) ===========

Index creation takes roughly 90 seconds.

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #330080