refactor: remove unused const and var to pass staticcheck

- Remove unused 'killed' const in util.go
- Remove unused 'command' var in examples/injection/main.go

examples/injection/main.go

@@ -7,13 +7,6 @@ import (
 	"github.com/taigrr/adb"
 )
 
-var command string
-
-func init() {
-	// TODO  allow for any input to be used as the command
-	command = "ls"
-}
-
 func main() {
 	ctx := context.TODO()
 	devs, err := adb.Devices(ctx)