fix: don't override behaviour of Home / End in pc keymap

Similarly removed in Prosemirror at ProseMirror/prosemirror-commands@20371c58

For users who wish to preserve this behaviour, they can achieve this via:

```javascript
const CustomKeyboardBehaviour = Extension.create({
  addKeyboardShortcuts() {
    return {
      ['Home']: () => this.editor.commands.selectTextblockStart(),
      ['End']: () => this.editor.commands.selectTextblockEnd(),
    }
  }
})

const editor = new Editor({
  extensions: [
    // Register your custom extension with the editor.
    CustomKeyboardBehaviour,
  ]
});
```

packages/core/src/extensions/keymap.ts

@@ -60,8 +60,6 @@ export const Keymap = Extension.create({
 
     const pcKeymap = {
       ...baseKeymap,
-      Home: () => this.editor.commands.selectTextblockStart(),
-      End: () => this.editor.commands.selectTextblockEnd(),
     }
 
     const macKeymap = {