[lworld] RFR: 8373608: [lworld] Remove ObjBufferAllocator

[Stefan Johansson]

Please review this change to remove `ObjBufferAllocator` since the impementation now is identical to `ObjAllocator`. 

**Summary**
This was not the case when ObjBufferAllocator was first introduced. Back then it skipped clearing the memory and just called oopDesc::set_klass_gap(mem, 0). This is no longer the case after [JDK-8350630](https://bugs.openjdk.org/browse/JDK-8350630) in which `ObjBufferAllocator` was changed to also clear the memory. The previous optimization is likely not needed and we should streamline the code to always use `ObjAllocator`.

**Testing**
* mach5 tier1-3 (ongoing)

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Commit messages:
 - 8373608: [lworld] Remove ObjBufferAllocator

Changes: https://git.openjdk.org/valhalla/pull/1794/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1794&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373608
  Stats: 34 lines in 7 files changed: 0 ins; 32 del; 2 mod
  Patch: https://git.openjdk.org/valhalla/pull/1794.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1794/head:pull/1794

PR: https://git.openjdk.org/valhalla/pull/1794