Remove PowerMock

[basil]

Without this PR, running the test suite with jenkins.version set to the latest weekly fails with:

java.lang.IllegalArgumentException: org.kohsuke.stapler.ClassLoaderValue$X referenced from a method is not visible from class loader
        at java.base/java.lang.reflect.Proxy$ProxyBuilder.ensureVisible(Proxy.java:858)
        at java.base/java.lang.reflect.Proxy$ProxyBuilder.validateProxyInterfaces(Proxy.java:681)
        at java.base/java.lang.reflect.Proxy$ProxyBuilder.<init>(Proxy.java:627)
        at java.base/java.lang.reflect.Proxy$ProxyBuilder.<init>(Proxy.java:635)
        at java.base/java.lang.reflect.Proxy.lambda$getProxyConstructor$0(Proxy.java:415)
        at java.base/jdk.internal.loader.AbstractClassLoaderValue$Memoizer.get(AbstractClassLoaderValue.java:329)
        at java.base/jdk.internal.loader.AbstractClassLoaderValue.computeIfAbsent(AbstractClassLoaderValue.java:205)
        at java.base/java.lang.reflect.Proxy.getProxyConstructor(Proxy.java:413)
        at java.base/java.lang.reflect.Proxy.getProxyClass(Proxy.java:384)
        at org.kohsuke.stapler.ClassLoaderValue.get(ClassLoaderValue.java:55)
        at org.kohsuke.stapler.MetaClassLoader.get(MetaClassLoader.java:53)
        at org.kohsuke.stapler.MetaClassLoader.<init>(MetaClassLoader.java:46)
        at org.jvnet.hudson.test.JenkinsRule.<clinit>(JenkinsRule.java:2878)
        ... 30 more

CC @res0nance

[basil]

Thanks @res0nance! If you're willing to do a release of this, that would facilitate PCT testing of this plugin in preparation for the forthcoming 2.361.1 LTS release.

[res0nance]

Thanks @res0nance! If you're willing to do a release of this, that would facilitate PCT testing of this plugin in preparation for the forthcoming 2.361.1 LTS release.

Done! Thanks for fixing this

[basil]

Thank you very much!