[code-reflection] Integrated: Removed hard coded jextract path

[Gary Frost]

This fix allows bld to access jextract from user PATH. Rather than hardcoding in the script.

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Commit messages:
 - Removed hard coded jextract path

Changes: https://git.openjdk.org/babylon/pull/293/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=293&range=00
  Stats: 154 lines in 2 files changed: 73 ins; 9 del; 72 mod
  Patch: https://git.openjdk.org/babylon/pull/293.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/293/head:pull/293

PR: https://git.openjdk.org/babylon/pull/293