RFR: 8300810: Get rid of unused JDI removeListener() methods

[Chris Plummer]

Remove removeListener() methods. They are not called.

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

Commit messages:
 - Remove removeListener() method.

Changes: https://git.openjdk.org/jdk/pull/12151/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12151&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300810
  Stats: 26 lines in 2 files changed: 0 ins; 24 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12151.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12151/head:pull/12151

PR: https://git.openjdk.org/jdk/pull/12151