Revert Mac Run Cell Keybinding Change in v6.1

**Describe the bug**
After updating my Jupyter Notebook, cmd+enter is now run cells in addition to ctrl+enter. I've been using Sublime key mappings where cmd+enter creates a new line below and moves my cursor there. I can no longer make that work and I am very frustrated.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

**Expected behavior**
A clear and concise description of what you expected to happen.

**Screenshots**
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**Desktop (please complete the following information):**
- OS: [e.g. iOS]
- Browser [e.g. chrome, safari]
- Version [e.g. 22]

**Additional context**
Add any other context about the problem here.

The issue reports that cmd+enter runs cells on Mac in addition to ctrl+enter and requests reverting that keybinding change. No file, test, or entry point is named, so first locate the Mac keyboard shortcut handling and reproduce the behavior; done means cmd+enter no longer runs a cell while ctrl+enter continues to do so.

Written by the indexing model from the issue text.