Reland "Enforce --skylab for "atest server"."

This reverts commit 44fbd139094a2f03ff828ad488bea00957c9609d.

Reason for revert: metrics are now using skylab inventory

Original change's description:
> Revert "Enforce --skylab for "atest server"."
>
> This reverts commit 38c18eb01f6935f16a80d5f536ec621b06047e0e.
>
> Reason for revert: sysmon not fixed yet, broke in staging
>
> Original change's description:
> > Enforce --skylab for "atest server".
> >
> > Enable --skylab by default for "atest server" and delete unused
> > list format options.
> >
> > BUG=chromium:832309
> > TEST="atest server commands"
> >
> > Change-Id: I257add48ac2a13d81f25a385178739c01df2eb6f
> > Reviewed-on: https://chromium-review.googlesource.com/1087506
> > Reviewed-by: Ningning Xia <[email protected]>
> > Tested-by: Ningning Xia <[email protected]>
>
> Bug: chromium:832309
> Change-Id: I28b70f08ca85c93c0cae33b909e918a7c007df49
> Reviewed-on: https://chromium-review.googlesource.com/1097746
> Reviewed-by: Allen Li <[email protected]>
> Reviewed-by: Prathmesh Prabhu <[email protected]>
> Commit-Queue: Allen Li <[email protected]>
> Tested-by: Allen Li <[email protected]>

Bug: chromium:832309
Change-Id: I9042da968b78fd1c1c0fac08148e72b5086940f6
Reviewed-on: https://chromium-review.googlesource.com/1163965
Commit-Ready: Allen Li <[email protected]>
Tested-by: Allen Li <[email protected]>
Reviewed-by: Allen Li <[email protected]>
diff --git a/cli/skylab_utils.py b/cli/skylab_utils.py
index bb76be6..5fb7c6a 100644
--- a/cli/skylab_utils.py
+++ b/cli/skylab_utils.py
@@ -27,7 +27,7 @@
         r'https://chrome-internal-review.googlesource.com/c/chromeos/'
         'infra_internal/skylab_inventory/\\+/([0-9]*)')
 MSG_INVALID_IN_SKYLAB = 'This is currently not supported with --skylab.'
-MSG_ONLY_VALID_IN_SKYLAB = 'This is only supported with --skylab.'
+MSG_ONLY_VALID_IN_SKYLAB = 'This only applies to actions on skylab inventory.'
 
 
 class SkylabInventoryNotImported(Exception):