Revert "[Enrollment] Fail closed in case of missing serial number"

This reverts commit 3bc5c3218edf2d097a9e957417cf30a85f32988b.

Reason for revert: blocks testing flow b/376252857

Original change's description:
> [Enrollment] Fail closed in case of missing serial number
>
> We can fail to obtain the serial number or brand code.
> Without this data we cannot perform enrollment state determination.
> Before this CL, we broke FRE by assuming "No Enrollment".
>
> This CL turns missing serial number or brand code into a blocker for
> OOBE.
>
> This also fixes a number of tests that were implicitly expecting
> "No enrollment" in enrollment state determination. They are now setting
> the respective command line switch explicitly.
>
> Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
> Bug: 373550440
> Test: Adjusted unit tests
> Change-Id: Ic8229b33e7e6a65d17a09310ef4fb4b5c9372109
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5939698
> Reviewed-by: Artem Sumaneev <asumaneev@google.com>
> Reviewed-by: Osama Fathy <osamafathy@google.com>
> Commit-Queue: Roland Bock <rbock@google.com>
> Reviewed-by: Victor Gabriel Savu <vsavu@google.com>
> Cr-Commit-Position: refs/heads/main@{#1373914}

Bug: 373550440
Change-Id: If94be9a2ad8c1f6be39e8c72a5e7f8a06c86dd93
Cq-Include-Trybots: luci.chrome.try:linux-chromeos-chrome
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5972389
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Roland Bock <rbock@google.com>
Reviewed-by: Renato Silva <rrsilva@google.com>
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Reviewed-by: Victor Gabriel Savu <vsavu@google.com>
Auto-Submit: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/main@{#1375771}