Remove misleading Mailbox::expunge()

[Slamdunk]

Consider this code:

$mailboxOne = $connection->getMailbox('one');
$mailboxTwo = $connection->getMailbox('two');

$mailboxOne->getMessages()[0]->delete();
$mailboxTwo->getMessages()[0]->delete();

$mailboxOne->expunge();

A user can think that only the message of the first mailbox is deleted, while in reality both are deleted.