Remove dgc interval settings.

Old JVM versions ran dgc by default every minute.
Increasing it to 10 minutes was common sense.

Since Java 6 the default value is 1 hour. Since we
now require Java 6, our setting in fact shortens
the default interval. It should be OK to simply
stick to the new Java 6 default.


git-svn-id: https://svn.apache.org/repos/asf/jmeter/trunk@1507721 13f79535-47bb-0310-9956-ffa450edef68