TASK: remove our behat `@Isolated` hack

[mhsdesign]

With https://github.com/neos/flow-development-collection/issues/3169 we cleaned up our behat testing infrastructure. The concept with @isolated tests is utterly complex. Neos doesnt use this feature anymore and we only have it in place for our two policy security features.

Running those two smaller fixtures via behat as we do right now adds massive complexity to our testing stack.

So id like to find alternative ways to @Isolated and remove this complexity.

This behat hack was introduced via b371bc8fcbbcb184c1b21d7a2cd9ec056464215b

[mhsdesign]

With https://github.com/neos/flow-development-collection/pull/3173 we declared this trait / infrastructure to be fully internal and deprecated. If we find an alternative way to test those two (small) features:

https://github.com/neos/flow-development-collection/blob/ad4d67abc9d3e9cef2939968c20e67e6785d326e/Neos.Flow/Tests/Behavior/Features/Security/Policy.method.feature https://github.com/neos/flow-development-collection/blob/ad4d67abc9d3e9cef2939968c20e67e6785d326e/Neos.Flow/Tests/Behavior/Features/Security/Policy.methodWithParameters.feature

we can remove the infrastructure with Neos 9.1