RFR: 8057776 Misc cleanups of the attach code

[Staffan Larsen]

Please review this simple fix for some small issues in the attach code:
- Removed unused imports
- Removed unused fields
- Update @since for startManagementAgent() and startLocalManagementAgent() since these were just backported to 1.8
- Simplified a for-loop in a test

bug: https://bugs.openjdk.java.net/browse/JDK-8057776
webrev: http://cr.openjdk.java.net/~sla/8057776/webrev.00/

Thanks,
/Staffan