ORC: use =destroy instead of =dispose (#18440)

* ORC refactoring in preparation for further changes (=dispose must die)
* ORC: embrace =destroy, avoid =dispose
* ORC: no need for =dispose
* closes #18421

lib/system.nim

@@ -1779,7 +1779,6 @@ when not defined(js) and defined(nimV2):
       align: int
       name: cstring
       traceImpl: pointer
-      disposeImpl: pointer
       typeInfoV1: pointer # for backwards compat, usually nil
       flags: int
     PNimTypeV2 = ptr TNimTypeV2