[Fuchsia] Turn-off multiplexed connections in SSH config

Multiplexed connections currently do not work well because of select()
restrictions on Fuchsia (see ZX-1555): each session adds at least 3
pipes that sshd needs to select on, so it hits 16 FDs limit very easily.
Second problem is that sshd doesn't know how to create new jobs for each
session and since there is no SIGHUP and won't kill child processes when
the client disconnects.

Bug: 778467
Change-Id: I1e6332a817b26dd80e3fe944b51f3ead1cb6c1d8
Reviewed-on: https://chromium-review.googlesource.com/865523
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#529494}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 62157460a07ed38f23d4ee24146205306ca9d1a1

fuchsia/runner_v2/boot_image.py

@@ -20,9 +20,6 @@ Host *
   User fuchsia
   IdentitiesOnly yes
   IdentityFile {identity}
-  ControlPersist yes
-  ControlMaster auto
-  ControlPath /tmp/fuchsia--%r@%h:%p
   ServerAliveInterval 1
   ServerAliveCountMax 1"""