Phonet: kill the ST-Ericsson pipe controller Kconfig

This is now a run-time choice so that a single kernel can support both
old and new generation ISI modems. Support for manually enabling the
pipe flow is removed as it did not work properly, does not fit well
with the socket API, and I am not aware of any use at the moment.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>