[JENKINS-25348]

Avoid having multiple copies of groovy-sandbox.jar in the Jenkins JVM,
which compromises the security.

I think it makes sense for the script-security plugin to own this
library, so instead let's depend on that plugin to provide a canonical
copy.

There's still a separate fix that needs to happen in script-security
plugin and its client plugins, but in the mean time this fix plugs the
hole as email-ext-plugin is the only other plugin in the jenkinsci org
that brings its own copy of groovy-sandbox