cmd/geth: drop upgradedb subcommand since it's unfeasible

This command was meant as a hackish way to upgrade our chain database way back
when nobody cared for live updates and the size of the database along with its
import times was small. With the current database weighing hundreds of GBs and
processing times of many days, this command is just ludicrous.

cmd/geth/main.go

@@ -65,7 +65,6 @@ func init() {
 		initCommand,
 		importCommand,
 		exportCommand,
-		upgradedbCommand,
 		removedbCommand,
 		dumpCommand,
 		// See monitorcmd.go: