Bulk cleanup of hudson.slaves and jenkins.slaves (#3207)

* Bulk cleanup of hudson.slaves and jenkins.slaves

- [x] Use Jenkins.get() instead of the deprecated getInstance()
- [x] Remove unused imports
- [x] Use diamond type definitions where possible
- [x] Squash similar extension handlers
- [x] Other minor refactorings and NPE checks

* Update core/src/main/java/hudson/slaves/SlaveComputer.java