RFR (XS) 8212025: Remove collector_present variable from ThreadHeapSampler

[JC Beyler]

Hi all,

Here is a tiny webrev to remove the _collectors_present member that is now
unused. I forgot to remove it when I did the fix for JDK-8211950.

Webrev: http://cr.openjdk.java.net/~jcbeyler/8212025/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8212025

My apologies,
Jc
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