RFR: 8266295: Remove unused _concurrent_iteration_safe_limit

[Albert Mingkun Yang]

Small change of removing an unused field; possible leftover of CMS.

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

Commit messages:
 - limit

Changes: https://git.openjdk.java.net/jdk/pull/3795/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3795&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266295
  Stats: 22 lines in 4 files changed: 0 ins; 21 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3795.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3795/head:pull/3795

PR: https://git.openjdk.java.net/jdk/pull/3795