JDK-8233220: Remove Space::_par_seq_tasks member as it was only used by CMS

[Ivan Walulya]

Hi all,

Please review a minor enhancement to remove Space::_par_seq_tasks member which was only used by CMS.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233220 <https://bugs.openjdk.java.net/browse/JDK-8233220>
Webrev: http://cr.openjdk.java.net/~lkorinth/ivan/8233220/ <http://cr.openjdk.java.net/~lkorinth/ivan/8233220/>


Testing: Tier 1 - Tier 3

//Ivan