Remove obsolete PropertyEditorManager cleanup task

The `registry` field no longer exists since https://github.com/openjdk/jdk/commit/d46ed5c358e5a0b0643ea929ce90cdc03a4a263d. Neither the IDE nor WindowBuilder support such ancient Java versions and this is therefore dead code.

https://github.com/eclipse-windowbuilder/windowbuilder/blob/a91864d0bead261a09d177d0d172f0b59771697b/org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/utils/state/EditorState.java#L254-L276

Open org.eclipse.wb.core.java/src/org/eclipse/wb/internal/core/utils/state/EditorState.java around lines 254-276 and inspect the cleanup task alongside the referenced OpenJDK change. Remove the obsolete task, then verify that the Java project still builds and that no remaining references to the removed registry field are present.

Written by the indexing model from the issue text.