Remove "turn device screen on" feature

Only keep "turn device screen off" and POWER button.

After we turn the device screen off (with Ctrl+o), turning it back on
does not always work, and leaves the device in a weird state, where even
the power button may not be sufficient:
<https://github.com/Genymobile/scrcpy/issues/175#issuecomment-497946596>

This is not an acceptable behavior, so disable the shortcut to turn the
physical device screen on. We can use the POWER button (or Ctrl+p)
instead.

README.md

@@ -286,7 +286,6 @@ you are interested, see [issue 14].
  | click on `POWER`                       | `Ctrl`+`p`                    |
  | power on                               | _Right-click²_                |
  | turn device screen off (keep mirroring)| `Ctrl`+`o`                    |
- | turn device screen on                  | `Ctrl`+`Shift`+`o`            |
  | expand notification panel              | `Ctrl`+`n`                    |
  | collapse notification panel            | `Ctrl`+`Shift`+`n`            |
  | copy device clipboard to computer      | `Ctrl`+`c`                    |