RFR: 8253244: Shenandoah: cleanup includes in Shenandoah root processor files

[Zhengyu Gu]

Many roots were moved to oop storages, Shenandoah root processor no longer references to StringTable, systemDictionary
and ResolvedMethodTable, etc.

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Commit messages:
 - JDK-8253244

Changes: https://git.openjdk.java.net/jdk/pull/206/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=206&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253244
  Stats: 7 lines in 2 files changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/206.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/206/head:pull/206

PR: https://git.openjdk.java.net/jdk/pull/206