usb_updater: stop supporting protocol versions below 5
usb_updater: stop supporting protocol versions below 5

There are no early H1 chips left in circulation, all released chips
(starting with RW 0.0.10) support update protocol version 5.

This patch drops all technical debt associated with supporting earlier
protocol versions.

BRANCH=cr50
BUG=b:35580805
TEST=downgraded an H1 test board to ro 0.0.8 rw 0.0.9. Updated it to
     the latest image (ro 0.0.10 rw 0.0.18).

Change-Id: I28c9b0c597122c7aa602a88fb56f9c7bf04b9984
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/457500
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec