Remove rpi-update (fix #302)

According to the official RPI documentation rpi-update isn't a general
purpose tool.

https://raspberrypi.org/documentation/linux/kernel/updating.md

src/main.rs

@@ -570,12 +570,6 @@ fn run() -> Result<()> {
                 || linux::run_fwupdmgr(run_type),
                 config.no_retry(),
             )?;
-            execute(
-                &mut report,
-                "rpi-update",
-                || linux::run_rpi_update(sudo.as_ref(), run_type),
-                config.no_retry(),
-            )?;
         }
 
         if config.should_run(Step::Restarts) {