Remove Disruptive tests tag from e2e documentation

[SergeyKanzhelev]

Discussed before: kubernetes/kubernetes#135142 (comment)

Let's remove the Disruptive tag. It feels unnecessary as discussed kubernetes/kubernetes#135142 (comment). As far as I know nobody runs disruptive tests one-per-environment. So the Serial already covers the fact that the test should not run with others

/cc @pohly

/sig testing

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[pohly]

In kubernetes/kubernetes#135142 (comment), @BenTheElder explained why "Disruptive" can be useful: someone running tests against an existing, in-use cluster may want to filter out "disruptive" tests to avoid affecting the normal workload in the cluster. We don't need this in any of our Prow jobs because those are specifically for running tests, so "disruptive" is acceptable. We rely on "serial" instead to manage interference between tests.

Someone running "serial" tests in an in-use cluster may have to be prepared for additional flakiness because one reason for "serial" can be "this tests must not be disturbed", but at least the tests won't break anything else.

"disruptive" implies "serial" (i.e. both must be set), but not the other way around.

I would keep "disruptive".

[SergeyKanzhelev]

@BenTheElder how much do you think it is important to keep the informational "Disruptive" tag? Most serial jobs will break if any other activity is happening on the node. So all of them are "Disrupted". Not all Serial will Disrupt other workload, but in most cases they will. So it is really not that informative to have the "Disruptive" tag. I would think following this up with adding a human-readable note to "Serial" tag that explains why it is serial will help more than having a Disruptive tag.