Clean up node_block_list_alter()

Problem/Motivation

This is a followup issue for #1535868: Convert all blocks into plugins.

• node_block_list_alter() could use cleanup and refactoring.
• It includes legacy code responding to the arguments in the node path in order to hide blocks on the node creation page:
  

    if (arg(0) == 'node' && arg(1) == 'add' && arg(2)) {
      $node_add_arg = strtr(arg(2), '-', '_');
    }
  ...
        elseif (isset($node_add_arg) && isset($node_types[$node_add_arg])) {
          // This is a node creation page
          if (!isset($block_node_types[$block->module][$block->delta][$node_add_arg])) {
            // This block should not be displayed for this node type.
            unset($blocks[$key]);
            continue;
          }
  

• It has strange logic with regard to which blocks are excluded from display:
  

      if (!isset($block->theme) || !isset($block->status) || $block->theme != $theme_key || $block->status != 1) {
        // This block was added by a contrib module, leave it in the list.
        continue;
  

Proposed resolution

• Clean up node_block_list_alter() if it is not replaced with a visibility plugin.