RFR: 8335865: Shenandoah: Improve THP pretouch after JDK-8315923

[Neethu Prasad]

**Notes**
os::pretouch is now using madvice now when available and has a fall back to using vm page size [JDK-8315923](https://bugs.openjdk.org/browse/JDK-8315923)
Hence removing code that sets _pretouch_heap_page_size & _pretouch_bitmap_page_size  in Shenandoah.

**Testing**

* Ran test in Linux 5.10 and Linux 6.x and confirmed that there is no regression. I could not replicate the issue or performance improvement though. [add results]
* Ran [TestTransparentHugePageUsage](https://github.com/openjdk/jdk/commit/a65a89522d2f24b1767e1c74f6689a22ea32ca6a) for Shenandoah and verified that test passed
* Ran tier 1, tier 2 , tier1_gc_shenandoah, tier2_gc_shenandoah, tier3_gc_shenandoah and hotspot_gc_shenandoah.

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Commit messages:
 - 8335865: Shenandoah: Improve THP pretouch after JDK-8315923

Changes: https://git.openjdk.org/jdk/pull/20254/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20254&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335865
  Stats: 18 lines in 1 file changed: 0 ins; 17 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20254.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20254/head:pull/20254

PR: https://git.openjdk.org/jdk/pull/20254