RFR: 8305207: Calendar.aggregateStamp(int, int) return value can be simplified

[Justin Lu]

Small cleanup / tweak spotted in Calendar to improve readability.

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Commit messages:
 - Replace ternary with max()

Changes: https://git.openjdk.org/jdk/pull/13554/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305207
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13554.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13554/head:pull/13554

PR: https://git.openjdk.org/jdk/pull/13554