The only way to get rid of that quest is to complete the Leviathan Raid. In D2 there are a few persistent quests where you won't be able to delete them. So either ignore them, or complete the requirements to get them out. With that said, the vast majority of quests and bounties can be deleted when you don't want them.
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Okay :( there are 3/4 on my titan I want to get rid of that are locked. Just wanted to clean up the pursuits going into shadowkeep. I already did the bounties on my warlock and just don't need them on the other 2. Thank you.
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If they are locked the only way to clear them out is to complete the quest steps. The particular quest step you linked to is for Legend of Acrius which is an exotic shotgun, so you may wish to complete that one.