RFR: 8371675: ZGC: Remove leftover X VMOp symbols

[Axel Boldt-Christmas]

Trivial change to remove the VMOp symbols from the `VM_OPS_DO` macro which were left over after the removal of the non-generational ZGC. ([JDK-8335850](https://bugs.openjdk.org/browse/JDK-8335850))

* Testing
  * GHA

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

Commit messages:
 - 8371675: ZGC: Remove leftover X VMOp symbols

Changes: https://git.openjdk.org/jdk/pull/28252/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371675
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28252.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28252/head:pull/28252

PR: https://git.openjdk.org/jdk/pull/28252