Move code to the appropriate layer

StructureNodeKind is a type which is specific to a particular feature,
file_structure. It shouldn't be in the "code shared by all ide features"
part.

crates/ide_db/src/lib.rs

@@ -135,12 +135,6 @@ fn line_index(db: &dyn LineIndexDatabase, file_id: FileId) -> Arc<LineIndex> {
     Arc::new(LineIndex::new(&*text))
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
-pub enum StructureNodeKind {
-    SymbolKind(SymbolKind),
-    Region,
-}
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord)]
 pub enum SymbolKind {
     Const,