Revert "[Tether] Move MessageTransferOperation to HostConnection"

This reverts commit e620b7d10c0f4ece326791008c0ada38bb0a130f.

Reason for revert:
LUCI Bisection has identified this change as the cause of a test failure. See the analysis: https://ci.chromium.org/ui/p/chromium/bisection/test-analysis/b/5748464983998464

Sample build with failed test: https://ci.chromium.org/b/8748656820021875937
Affected test(s):
[ninja://chromeos:chromeos_unittests/ConnectTetheringOperationTest.ConnectRequestSentOnceAuthenticated](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fchromeos:chromeos_unittests%2FConnectTetheringOperationTest.ConnectRequestSentOnceAuthenticated?q=VHash%3Ab1d1122dca4f561e)
[ninja://chromeos:chromeos_unittests/ConnectTetheringOperationTest.GetMessageTimeoutSeconds](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fchromeos:chromeos_unittests%2FConnectTetheringOperationTest.GetMessageTimeoutSeconds?q=VHash%3Ab1d1122dca4f561e)
[ninja://chromeos:chromeos_unittests/TetherConnectorImplTest.TestNewConnectionAttemptDuringOperation_DifferentDevice](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fchromeos:chromeos_unittests%2FTetherConnectorImplTest.TestNewConnectionAttemptDuringOperation_DifferentDevice?q=VHash%3Ab1d1122dca4f561e)
[ninja://chromeos:chromeos_unittests/TetherConnectorImplTest.TestNewConnectionAttemptDuringWifiConnection_DifferentDevice](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fchromeos:chromeos_unittests%2FTetherConnectorImplTest.TestNewConnectionAttemptDuringWifiConnection_DifferentDevice?q=VHash%3Ab1d1122dca4f561e)
[ninja://chromeos:chromeos_unittests/TetherConnectorImplTest.TestSuccessfulConnection_SetupRequired](https://ci.chromium.org/ui/test/chromium/ninja:%2F%2Fchromeos:chromeos_unittests%2FTetherConnectorImplTest.TestSuccessfulConnection_SetupRequired?q=VHash%3Ab1d1122dca4f561e)

If this is a false positive, please report it at http://b.corp.google.com/createIssue?component=1199205&description=Analysis%3A+https%3A%2F%2Fci.chromium.org%2Fui%2Fp%2Fchromium%2Fbisection%2Ftest-analysis%2Fb%2F5748464983998464&format=PLAIN&priority=P3&title=Wrongly+blamed+https%3A%2F%2Fchromium-review.googlesource.com%2Fc%2Fchromium%2Fsrc%2F%2B%2F5386008&type=BUG

Original change's description:
> [Tether] Move MessageTransferOperation to HostConnection
>
> Change-Id: I78bc03e5fb5371929cb868013bea0757a5eb76f8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5386008
> Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
> Commit-Queue: Joe Antonetti <joeantonetti@google.com>
> Cr-Commit-Position: refs/heads/main@{#1297006}
>

Change-Id: Ied9999aeaf5816d88c5779879988d0f8b7f47bf1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5519969
Owners-Override: Yuki Shiino <yukishiino@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1297228}