faced this yesterday.. came to this thread.. later found solution and shared.. hope this helps someone ahead 🙂

Delete the plan using query-

SELECT

s.name AS [Name],

s.id AS [ID]

FROM

msdb.dbo.sysmaintplan_plans AS s

EXECUTE msdb.dbo.sp_maintplan_delete_plan @plan_id=N'{maintenance plan}'

GO

Worked for me too !! thanks!!