Remove "mosys platform name" command for unibuild

This command is not used in CrOS anymore for unibuild devices, so it
can be removed.

This requires removing the entire "platform" subcommand for unibuild,
as the last platform subcommand for unibuild was "name".

BUG=b:187790074
TEST=cros_workon_make --board caroline --test

Change-Id: I79aff9c503a526619145644f82984427aaf3ff70
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mosys/+/4177902
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Auto-Submit: Jack Rosenthal <jrosenth@chromium.org>
14 files changed