RFR: CODETOOLS-7903207: Simplify make/jtreg.gmk rules

[Jan Kratochvil]

CODETOOLS-7903207: Simplify make/jtreg.gmk rules

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Commit messages:
 - Simplify make/jtreg.gmk rules

Changes: https://git.openjdk.org/jtreg/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=86&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903207
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jtreg/pull/86.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/86/head:pull/86

PR: https://git.openjdk.org/jtreg/pull/86