pppd: Remove promptpass plugin

[paulusmack]

This is prompted by a number of factors:

• The handling of privileges and possibly file descriptors is flawed

• In many cases there will be no terminal accessible to the prompter program at the point where the prompter is run

• The passwordfd plugin does much the same thing but does it more cleanly and securely.

[Neustradamus]

@paulusmack: You have canceled your PR?

[paulusmack]

Yes, Jaco Kroon wanted me to be as vague as possible in the public commit, so I'll remove it along with other old stuff and not mention the bug in dropping privileges.