delete passphrases from InMemoryPassPhraseStorage on logout

[sosnovsky]

Before deleting I checked where removePassPhrases is used and it was used only in 1 place - https://github.com/FlowCrypt/flowcrypt-ios/blob/master/FlowCrypt/Functionality/Services/Key%20Services/PassPhraseService.swift#L86

So after removing EncryptedStorage I removed this method too, as it wasn't used anywhere else.
It seems currently we're not deleting passphrases from memory on logout. I'll rollback this change and call removePassPhrases on logout.

Originally posted by @sosnovsky in #577 (comment)