fastboot: remove 'powerdown'

This isn't sent by the fastboot client.
This isn't in our bootloader requirement document.
Neither my hikey nor sailfish respond to this.
The README.md literally uses this as an example for 'unknown command'

Test: fastboot still works.
Change-Id: Ia442a41de517dae078dd7e73c951aadb97646c73
5 files changed