Drop remove_on_close column from labels table in envs where it exists

What does this MR do?

Removes remove_on_close column from the labels table on envs where the original migration (from !61286 (merged)) was run. Migration was reverted before a new release was made.

Step 2 of 3 on column removal. Column was already ignored in a previous release (!62246 (merged))

Relates to #331414 (closed)

Migration output

UP when column exists

== 20210531164210 DropRemoveOnCloseFromLabels: migrating ======================
-- column_exists?(:labels, :remove_on_close)
   -> 0.0031s
-- remove_column(:labels, :remove_on_close)
   -> 0.0013s
== 20210531164210 DropRemoveOnCloseFromLabels: migrated (0.0103s) =============

UP when column does not exist

== 20210531164210 DropRemoveOnCloseFromLabels: migrating ======================
-- column_exists?(:labels, :remove_on_close)
   -> 0.0031s
== 20210531164210 DropRemoveOnCloseFromLabels: migrated (0.0032s) =============

DOWN

== 20210531164210 DropRemoveOnCloseFromLabels: reverting ======================
== 20210531164210 DropRemoveOnCloseFromLabels: reverted (0.0000s) =============

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have self-reviewed this MR per code review guidelines.
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Security

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Related to #331414 (closed)