Remove spurious queue add warning

[hjoliver]

Describe the bug

If a task foo inherits from two families A and B, assigning members of both A and B to a queue will result (on adding the second family) in a warning that foo has already been added to a queue. This is fine if A and B are separately added to different queues, but pointless at best if they are both added to the same queue.

https://cylc.discourse.group/t/suggestion-for-queue-warnings/190

Release version(s) and/or repository branch(es) affected?

7.8.x and current master

Steps to reproduce the bug

(Just reproduce the above scenario in a small workflow definition, then validate it).

Assigning myself as I already put in a quick fix while looking at the problem.

(PR should go to both 7.8.5 and 8.0a2)

[wxtim]

@hjoliver Can we get a PR or push this beyond 7.8.5?

[hjoliver]

I'll take a quick look now...

[hjoliver]

Closed by #3539 #3540 #3541