RFR: 8342255: GenShen: Remove unnecessary enum initial values

[William Kemper]

C++ does not require explicit initial values for enumeration members.

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Commit messages:
 - Remove unnecessary values for enumeration

Changes: https://git.openjdk.org/shenandoah/pull/514/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=514&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342255
  Stats: 12 lines in 1 file changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/shenandoah/pull/514.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/514/head:pull/514

PR: https://git.openjdk.org/shenandoah/pull/514