RFR: 8281971: Remove unimplemented InstanceRefKlass::do_next

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/7497/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7497&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281971
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7497.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7497/head:pull/7497

PR: https://git.openjdk.java.net/jdk/pull/7497