Fix IT2Me protocol error connecting to old hosts.

Fix IT2Me protocol error connecting to old hosts.

This is a revert of the client-side changes introduced in https://codereview.chromium.org/1799293002.
Pre-M51 IT2Me hosts do not support anything except spake2-plain, so M51 and later clients cannot
connect to them

BUG=607643
Review-Url: https://codereview.chromium.org/1944553002
Cr-Commit-Position: refs/heads/master@{#391140}
(cherry picked from commit 5c5aec85e2efd5777723c61b3b13b59f6f235331)

Committed: https://chromium.googlesource.com/chromium/src/+/eaf2e629ca99893cca93dfafe1e7e7338cd2907e

[Jamie, 2016-05-04 00:35:55]

Description was changed from

==========
Fix IT2Me protocol error connecting to old hosts.

This is a revert of the client-side changes introduced in
https://codereview.chromium.org/1799293002.
Pre-M51 IT2Me hosts do not support anything except spake2-plain, so M51 and
later clients cannot
connect to them

BUG=607643

Review-Url: https://codereview.chromium.org/1944553002
Cr-Commit-Position: refs/heads/master@{#391140}
(cherry picked from commit 5c5aec85e2efd5777723c61b3b13b59f6f235331)
==========

to

==========
Fix IT2Me protocol error connecting to old hosts.

This is a revert of the client-side changes introduced in
https://codereview.chromium.org/1799293002.
Pre-M51 IT2Me hosts do not support anything except spake2-plain, so M51 and
later clients cannot
connect to them

BUG=607643

Review-Url: https://codereview.chromium.org/1944553002
Cr-Commit-Position: refs/heads/master@{#391140}
(cherry picked from commit 5c5aec85e2efd5777723c61b3b13b59f6f235331)

Committed:
https://chromium.googlesource.com/chromium/src/+/eaf2e629ca99893cca93dfafe1e7...
==========

[Jamie, 2016-05-04 00:35:56]

Committed patchset #1 (id:1) manually as
eaf2e629ca99893cca93dfafe1e7e7338cd2907e.

[Jamie, 2016-05-10 00:30:05]

+sergeyu FHI.