RFR: JDK-8311249: Remove unused MemAllocator::obj_memory_range

[Aleksey Shipilev]

This is a leftover from [JDK-8171221](https://bugs.openjdk.org/browse/JDK-8171221), which removed its only use.
Gets into the way of [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457).

Additional testing:
 - [ ] GHA builds

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/14759/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14759&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311249
  Stats: 15 lines in 2 files changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14759.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14759/head:pull/14759

PR: https://git.openjdk.org/jdk/pull/14759