refactor(editor): remove unused code (#12142)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

- **Refactor**
  - Streamlined internal logic for handling block operations to improve efficiency.
  - Removed unused reactive subject and related code from document handling classes.
  - Renamed a private initialization method for improved clarity.

- **Chores**
  - Cleaned up redundant code and imports to simplify maintenance.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/modules/workspace/impls/doc.ts

@@ -9,7 +9,6 @@ import {
   type Workspace,
   type YBlock,
 } from '@blocksuite/affine/store';
-import { Subject } from 'rxjs';
 import { Awareness } from 'y-protocols/awareness.js';
 import * as Y from 'yjs';
 
@@ -24,7 +23,7 @@ export class DocImpl implements Doc {
 
   private readonly _storeMap = new Map<string, Store>();
 
-  private readonly _initSubDoc = () => {
+  private readonly _initSpaceDoc = () => {
     {
       // This is a piece of old version compatible code. The old version relies on the subdoc instance on `spaces`.
       // So if there is no subdoc on spaces, we will create it.
@@ -47,9 +46,6 @@ export class DocImpl implements Doc {
 
   private _loaded!: boolean;
 
-  // eslint-disable-next-line rxjs/finnish
-  private readonly _onLoadSlot = new Subject();
-
   /** Indicate whether the block tree is ready */
   private _ready = false;
 
@@ -104,7 +100,7 @@ export class DocImpl implements Doc {
   constructor({ id, collection, doc }: DocOptions) {
     this.id = id;
     this.rootDoc = doc;
-    this._ySpaceDoc = this._initSubDoc() as Y.Doc;
+    this._ySpaceDoc = this._initSpaceDoc() as Y.Doc;
     this.awarenessStore = new AwarenessStore(new Awareness(this._ySpaceDoc));
 
     this._yBlocks = this._ySpaceDoc.getMap('blocks');
@@ -127,7 +123,6 @@ export class DocImpl implements Doc {
   private _destroy() {
     this.awarenessStore.destroy();
     this._ySpaceDoc.destroy();
-    this._onLoadSlot.unsubscribe();
     this._loaded = false;
   }