RFR: 8256419: ZGC: Remove "Heap Used Before/After Mark/Relocation" samplers

[Per Liden]

I propose that we remove the following stat samplers:

- Heap Used Before Mark
- Heap Used After
- Heap Used Before Relocation
- Heap Used After Relocation

The simple reason is that they aren't very useful. Looking at the heap statistics table gives a much better overview of what the heap usage has looked like.

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

Commit messages:
 - 8256419: ZGC: Remove "Heap Used Before/After Mark/Relocation" samplers

Changes: https://git.openjdk.java.net/jdk/pull/1239/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1239&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256419
  Stats: 10 lines in 1 file changed: 0 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1239.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1239/head:pull/1239

PR: https://git.openjdk.java.net/jdk/pull/1239