RFR: 8264040: Simplify the partial obj size calculation in ParallelCompactData::add_obj

[Albert Mingkun Yang]

Remove unnecessary "off by one" adjustment.

-------------

Commit messages:
 - fill

Changes: https://git.openjdk.java.net/jdk/pull/3160/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3160&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264040
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3160.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3160/head:pull/3160

PR: https://git.openjdk.java.net/jdk/pull/3160