whisper: flaky v5 test

[nolash]

@gballet This test has been flaking on Travis for nearly a year to my reckoning:

?       github.com/ethereum/go-ethereum/whisper/shhclient    [no test files]
--- FAIL: TestSimulation (12.28s)
    peer_test.go:196: Test was not complete: timeout 10 seconds.
FAIL

https://travis-ci.org/ethereum/go-ethereum/jobs/477291221

I briefly ran this by @gluk256 and he said this belongs to v5 which by now perhaps is obsolete. If so, maybe the test is obsolete too, and should be skipped?

[gballet]

Funny, I've never seen this one fail in travis. I'll have a look into it thanks. I don't want to remove a test, but we should remove v5 altogether. I will warn again and remove it.

[nolash]

Ok. However please keep in mind that pss uses v5 as library (not the network, so the sim is not relevant for this). Our v6 port would have to be done prior to removing it.

[gballet]

I am aware, which is why I requested your review in #18432. Please review it when you get a chance.

[gballet]

If you believe that my fix isn't going to work, we can also remove everything but what you need, but I would prefer not having dangling bits of code.

[nonsense]

FYI this is still failing on master occasionally.

[gballet]

Whisper v5 has been removed on March the 1st, closing.