RFR: 8281637: Remove unused VerifyOption_G1UseNextMarking

[Stefan Johansson]

Please review this change to remove an unused verification option.

**Summary**
In G1 there have been three different `VerifyOption` that can be passed to verification to signal how to determine liveness. One of these options, `VerifyOption_G1UseNextMarking`,  has been unused for quite some time and this change removes it. When removing it the `G1CollectedHeap::is_obj_ill(...)` functions are more or less unused, so I removed those as well and inlined the checks in the only place where it is really used `G1CMIsAliveClosure::do_object_b(...)`

**Testing**

- [x] Mach5
- [x] Local verification testing

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Commit messages:
 - 8281637: Remove unused VerifyOption_G1UseNextMarking

Changes: https://git.openjdk.java.net/jdk/pull/7440/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7440&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281637
  Stats: 66 lines in 9 files changed: 13 ins; 48 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7440.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7440/head:pull/7440

PR: https://git.openjdk.java.net/jdk/pull/7440