RFR: 8285364: Remove REF_ enum for java.lang.ref.Reference [v5]

[Albert Mingkun Yang]

> Simple rename and some comments update.
> 
> Test: build

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8332/files
  - new: https://git.openjdk.java.net/jdk/pull/8332/files/e4f6a66a..4b9081be

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8332&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8332&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8332.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8332/head:pull/8332

PR: https://git.openjdk.java.net/jdk/pull/8332