RFR (xs): 8138846: Remove unused enum ConcurrentGCThread::CGC_flag_type

[Per Liden]

Hi,

Could I have a couple of reviews on this small cleanup. While working on 
an unrelated change I noticed this unused enum CGC_flag_type in 
ConcurrentGCThread. This patch removes the enum and the related set/get 
functions. It also removes the the empty destructor, which is there for 
no good reason (and has an incorrect comment attached to it).

Webrev: http://cr.openjdk.java.net/~pliden/8138846/webrev.0/

Bug: https://bugs.openjdk.java.net/browse/JDK-8138846

/Per