Correct layouts/default/readme.md

[dunk2k]

Description

remove 60_ansi_arrow_split_bs_7u_spc diagram; removal originally approved and merged in #23259, reverted when merge of #22556 (merged after the former mentioned PR) was applied to develop branch.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• mismatch in layouts/ dir

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).