Delete accidentallly tracked *.ipynb files

plot_all_note_durations.ipynb

    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.5"
+   "version": "3.8.6"
   }
  },
  "nbformat": 4,

plot_calibrate.ipynb

    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.5"
+   "version": "3.8.6"
   }
  },
  "nbformat": 4,

plot_us_db_range.ipynb

    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.5"
+   "version": "3.8.6"
   }
  },
  "nbformat": 4,