RFR: 8315850: Improve AbstractMap anonymous Iterator classes [v2]

[Per Minborg]

> This PR proposes to slightly improve some iterators of `AbstractMap`:
> 
> * Code reuse
> * A field declared `final`
> * Add missing `@Override` annotations

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Remove @Override annotations

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15615/files
  - new: https://git.openjdk.org/jdk/pull/15615/files/1b00c84b..aab2472a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15615&range=00-01

  Stats: 10 lines in 1 file changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15615/head:pull/15615

PR: https://git.openjdk.org/jdk/pull/15615