RFR: 8255837: Shenandoah: Remove ShenandoahConcurrentRoots class

[Zhengyu Gu]

The class was introduced for 2 purposes:
1) a platform supports concurrent class unloading (e.g. the platform supports nmethod_entry_barrier) 
2) should perform concurrent class unloading for particular gc cycle (e.g. STW vs. concurrent GC)

Now, concurrent class unloading is supported on all Shenandoah supported platforms. Furthermore, STW and concurrent GC are isolated (JDK-8255765), the class becomes superfluous.

Test:
- [x] hotspot_gc_shenandoah Linux x86_64 and x86_32
- [x] nightly

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Commit messages:
 - Fixed copyright years
 - Fixed merge issue
 - Update
 - Merge
 - ClassUnloading -> heap->unload_classes()
 - JDK-8255837
 - Merge master
 - Merge branch 'JDK-8255765-isolate-gcs' into JDK-8256298-conc-stack-proc
 - Fixed indentation
 - More from Aleksey's review
 - ... and 127 more: https://git.openjdk.java.net/jdk/compare/fd00ed74...172438ec

Changes: https://git.openjdk.java.net/jdk/pull/2262/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2262&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255837
  Stats: 165 lines in 16 files changed: 0 ins; 124 del; 41 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2262.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2262/head:pull/2262

PR: https://git.openjdk.java.net/jdk/pull/2262