Remove snap refresh/revert tests from ubuntucore automated test plan

In a comment of the PR[1], Patrick explained the tests are meant for
Certification more than pre-GM testing, and therefore should be removed
from the ubuntucore-automated test plan:

> Recently, we QA has done a few milestone tests with Checkbox v2.10.1
> which has the kernel refresh/revert jobs landed. We ran into a
> situation where the pre-GM milestone kernel was not in the
> latest/stable channel due to various kinds of reasons. such as some
> SKUs would like to share the same kernel in the end, but for the
> kernel development, they place the kernel in different branches for
> alpha or beta ... milestones.
>
> It would cause some trouble for QA when the job wants to refresh the
> kernel to latest/stable because the revision will be different in the
> situation above.

[1]
#724 (comment)

providers/base/units/ubuntucore/test-plan.pxu

@@ -11,15 +11,7 @@ id: ubuntucore-automated
 unit: test plan
 _name: Automated Ubuntu Core OS feature tests
 _description: Automated OS feature tests for Ubuntu Core devices
-bootstrap_include:
-    snap_revision_info
 include:
-    ubuntucore/snap-refresh-kernel-.*-stable-rev
-    ubuntucore/reboot-after-snap-refresh-kernel-.*-stable-rev
-    ubuntucore/snap-verify-after-refresh-kernel-.*-stable-rev
-    ubuntucore/snap-revert-kernel-.*-stable-rev
-    ubuntucore/reboot-after-snap-revert-kernel-.*-stable-rev
-    ubuntucore/snap-verify-after-revert-kernel-.*-stable-rev
 
 id: ubuntucore-manual
 unit: test plan