unsetting ASK_PASS environment variable to disable x11 password window

Solves ipython/ipython#5154 .
(If ssh needs a passphrase and - there is no tty - the DISPLAY environment variable is set - the SSH_ASKPASS environment variable is set, it will open an X11 window to read the passphrase.)
This is useful to connect to a remote kernel with password authentication from a graphical IDE like Spyder.