Clear block type when pressing backspace

Summary:
This is closer to how most editors work.
Closes #1659

Reviewed By: sophiebits

Differential Revision: D7073440

Pulled By: sophiebits

fbshipit-source-id: 3f6ab214282878dd7dccf7709fc261ffe350f485

Author: sophiebits

src/model/modifier/RichTextEditorUtil.js

@@ -367,9 +367,8 @@ const RichTextEditorUtil = {
   },
 
   /**
-   * When a collapsed cursor is at the start of the first styled block, or
-   * an empty styled block, changes block to 'unstyled'. Returns null if
-   * block or selection does not meet that criteria.
+   * When a collapsed cursor is at the start of a styled block, changes block
+   * type to 'unstyled'. Returns null if selection does not meet that criteria.
    */
   tryToRemoveBlockStyle: function(editorState: EditorState): ?ContentState {
     var selection = editorState.getSelection();
@@ -379,11 +378,6 @@ const RichTextEditorUtil = {
       var content = editorState.getCurrentContent();
       var block = content.getBlockForKey(key);
 
-      var firstBlock = content.getFirstBlock();
-      if (block.getLength() > 0 && block !== firstBlock) {
-        return null;
-      }
-
       var type = block.getType();
       var blockBefore = content.getBlockBefore(key);
       if (