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.

assets/js/front-submit-editor-basics.js

@@ -48,9 +48,8 @@
     /**
      * 正文自身的副本：剔除 TinyMCE 的辅助节点。
      *
-     * 编辑器会把块级删除浮层（#jio-block-actions）之类带文字的工具节点插进
-     * body，它们都带 data-mce-bogus。直接读 body.textContent 会把「选中此块」
-     * 当成正文，导致空正文永远判成非空、占位符不显示。
+     * 编辑器会把带文字的工具节点插进 body，它们都带 data-mce-bogus。直接读
+     * body.textContent 会把工具文案当成正文，导致空正文永远判成非空、占位符不显示。
      */
     function contentClone(body) {
       var clone = body.cloneNode(true);