RFR: Cleanup unused _decompile and _ignore_decompile fields in AOTCodeEntry

[Ashutosh Mehra]

New workflow always sets `_ignore_decompile` to true, which implies `AOTCodeEntry::_decompile` is not in `check_entry` and is thus redundant. These two fields can just be removed.

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Commit messages:
 - Cleanup unused code

Changes: https://git.openjdk.org/leyden/pull/80/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=80&range=00
  Stats: 87 lines in 2 files changed: 3 ins; 32 del; 52 mod
  Patch: https://git.openjdk.org/leyden/pull/80.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/80/head:pull/80

PR: https://git.openjdk.org/leyden/pull/80