DOC: Remove pipe mode reference

Pipe mode was removed in 2.0, remove reference to it in the documentation.

Thanks to Piotr Wydrych for the bugreport.
32 jobs for <span class="ref-name">int-new</span> in 4 minutes and 2 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Image
passed #166212
dind allowed to fail
docker_centos-6-amd64

00:00:18

passed #166213
dind allowed to fail
docker_centos-7-amd64

00:00:18

passed #166202
dind allowed to fail
docker_debian-7-amd64

00:00:20

passed #166206
dind allowed to fail
docker_debian-7-i386

00:00:18

passed #166203
dind allowed to fail
docker_debian-8-amd64

00:00:20

passed #166207
dind allowed to fail
docker_debian-8-i386

00:00:16

passed #166204
dind allowed to fail
docker_debian-9-amd64

00:00:19

passed #166208
dind allowed to fail
docker_debian-9-i386

00:00:18

passed #166205
dind allowed to fail
docker_debian-testing-amd64

00:00:19

passed #166209
dind allowed to fail
docker_debian-testing-i386

00:00:18

passed #166210
dind allowed to fail
docker_fedora-25-amd64

00:00:20

passed #166211
dind allowed to fail
docker_fedora-26-amd64

00:00:19

passed #166214
dind allowed to fail
docker_opensuse-42_3-amd64

00:00:19

passed #166215
dind allowed to fail
docker_ubuntu-14_04-amd64

00:00:17

passed #166216
dind allowed to fail
docker_ubuntu-16_04-amd64

00:00:18

 
  Build
passed #166223
docker linux amd64
build-centos-6-amd64

00:01:30

passed #166224
docker linux amd64
build-centos-7-amd64

00:01:05

passed #166217
docker linux amd64
build-debian-7-amd64

00:01:44

passed #166228
docker linux amd64
build-debian-7-i386

00:01:11

passed #166218
docker linux amd64
build-debian-8-amd64

00:01:49

passed #166229
docker linux amd64
build-debian-8-i386

00:01:52

passed #166219
docker linux amd64
build-debian-9-amd64

00:01:17

passed #166230
docker linux amd64
build-debian-9-i386

00:01:32

passed #166220
docker linux amd64
build-debian-testing-amd64

00:01:48

passed #166231
docker linux amd64
build-debian-testing-i386

00:01:51

passed #166221
docker linux amd64
build-fedora-25-amd64

00:01:47

passed #166222
docker linux amd64
build-fedora-26-amd64

00:01:14

passed #166232
freebsd amd64
build-freebsd-11-amd64

00:02:49

passed #166233
freebsd i386
build-freebsd-11-i386

00:02:51

passed #166225
docker linux amd64
build-opensuse-42_3-amd64

00:01:36

passed #166226
docker linux amd64
build-ubuntu-14_04-amd64

00:01:14

passed #166227
docker linux amd64
build-ubuntu-16_04-amd64

00:01:48