pppd: Remove passprompt plugin

pppd: Remove passprompt plugin

This is prompted by a number of factors:

* It was more useful back in the dial-up days, but no-one uses dial-up
  any more

* 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

* The handling of privileges and file descriptors needs to be audited
  thoroughly.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>