RFR: 8290290: Remove addition of TimeInstants

[Kim Barrett]

Please review this trivial change to remove the unused and semantically suspect
function TimeInstant::operator+(TimeInstant).

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

Commit messages:
 - remove addition of TimeInstants

Changes: https://git.openjdk.org/jdk/pull/9489/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9489&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290290
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9489.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9489/head:pull/9489

PR: https://git.openjdk.org/jdk/pull/9489