RFR: 8296734: JarVerifier:: mapSignersToCodeSource should cache in map [v4]

[pandaapo]

> The cache named `signerToCodeSource` in `JarVerifier` is never used now.

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

  Modify as reviews.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11072/files
  - new: https://git.openjdk.org/jdk/pull/11072/files/ef4ba3df..c4cfb499

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11072&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11072&range=02-03

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11072.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11072/head:pull/11072

PR: https://git.openjdk.org/jdk/pull/11072