Remove alert styles

[janfaracik]

With jenkinsci/jenkins#9115 it's no longer necessary for themes to set alert style variables - so this merge request removes them.

Can either have the minimum Jenkins version be set to the weekly that contains the above MR or I can set the variables here with a comment that they can eventually be removed.

Before 9115

After 9115

Testing done

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[timja]

I've added a screenshot, I think update the minimum version is best for maintenance.

[janfaracik]

Updated it to point to the latest weekly with the relevant alert style changes.