This commit removes a misleading comment regarding
'zerver_message_edit_history_id' index.
We added the index in 0679 to use in 0680 but later the 0680 migration
was reworked resulting in the index not being used in 0680.
We didn't drop the index as we expect it to be helpful for other
things.
The comment was misleading hence removed.
68b7deac72