refactor(submit): drop the hover block-delete overlay

Hovering a quote, hidden block, code block or table showed a ✕ button in the
block's top-right corner. It read as a stray close button and is not wanted.

Removed
- installBlockActions() and the two-step select/delete flow it drove
- the block-actions markup, positioning, selected-state CSS in the iframe
  content style
- BLOCK_SELECTOR, which only the overlay used

Deleting blocks still works: removeAdjacentBlock() keeps Backspace / Delete
next to a structural block (table, pre, hr, quote, hidden block) working, and
that is covered by both code and tests.

tests/widget-system/submit-placeholder-behavior.test.cjs

@@ -140,13 +140,13 @@ test('placeholder never becomes a DOM node in the editable body', () => {
 });
 
 test('editor chrome injected into the body does not count as content', () => {
-  // 真实页面上 editor-runtime 会把块删除浮层插进 body，它带 data-mce-bogus
-  // 但含文字（「✕ 选中此块」）。若按 body.textContent 判断，空正文永远非空，
-  // 占位符就永远不显示 —— 这正是「首次加载不显示」的真实原因。
+  // TinyMCE 会往 body 里插带 data-mce-bogus 的工具节点，其中可能含文字。
+  // 若按 body.textContent 判断，空正文永远非空，占位符就永远不显示 ——
+  // 这正是「首次加载不显示」的真实原因。
   const { doc, editor } = createEditor(
     '<p><br data-mce-bogus="1"></p>' +
-      '<div id="jio-block-actions" data-mce-bogus="all">' +
-      '<button type="button"><span>✕</span><span>选中此块</span></button></div>',
+      '<div data-mce-bogus="all">' +
+      '<button type="button"><span>✕</span><span>工具节点</span></button></div>',
     { placeholder: '请输入正文' }
   );
   const plugin = loadPlugin()[PLUGIN];