RFR: 8254777: Remove unimplemented Management::get_loaded_classes

[Aleksey Shipilev]

There seems to be no definition of `Management::get_loaded_classes` anywhere in current tip or history prior the
initial load. Can be removed.

Testing:
 - [x] Linux x86_64 build
 - [x] Text search for `get_loaded_classes` in `src/hotspot`

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Commit messages:
 - 8254777: Remove unimplemented Management::get_loaded_classes

Changes: https://git.openjdk.java.net/jdk/pull/660/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=660&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254777
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/660.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/660/head:pull/660

PR: https://git.openjdk.java.net/jdk/pull/660