[autotest] Remove some dead code from server/hosts.

This removes dead code from server related to selecting which kernel
to boot during test jobs.  This doesn't remove all references to
code for this feature:
  * There is dead code that could trigger the code deleted here (if
    it weren't dead).
  * This change creates more unreferenced code that has yet to be
    deleted.

BUG=chromium:266704
TEST=unit tests, run in a local instance, grep for dangling references.

Change-Id: I706f46ca51bfa7e5455c8cf4d535bfbd09c6ed21
Reviewed-on: https://chromium-review.googlesource.com/348830
Commit-Ready: Richard Barnette <jrbarnette@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/server/hosts/servo_host.py b/server/hosts/servo_host.py
index faaadd2..f2772c2 100644
--- a/server/hosts/servo_host.py
+++ b/server/hosts/servo_host.py
@@ -446,8 +446,7 @@
             # don't want servo failure to fail the test with error:
             # `Host did not return from reboot` in status.log.
             reboot_cmd = 'sleep 1 ; reboot & sleep 10; reboot -f',
-            self.reboot(reboot_cmd=reboot_cmd, fastsync=True,
-                        label=None, wait=False)
+            self.reboot(reboot_cmd=reboot_cmd, fastsync=True, wait=False)
 
             # We told the reboot() call not to wait, but we need to wait
             # for the reboot before we continue.  Alas.  The code from
@@ -619,9 +618,8 @@
     def _repair_with_sysrq_reboot(self):
         """Reboot with magic SysRq key."""
         self.reboot(timeout=self.REBOOT_TIMEOUT_SECS,
-                    label=None,
                     down_timeout=self.HOST_DOWN_TIMEOUT_SECS,
-                    reboot_cmd='echo "b" > /proc/sysrq-trigger &',
+                    reboot_cmd='echo "b" > /proc/sysrq-trigger',
                     fastsync=True)
         time.sleep(self.REBOOT_DELAY_SECS)