remove unused variable

Follow-up to 3c0b5b9, remove this variable. It's not used anymore.

Change-Id: I390d694cf5650d137763391717c39677537526d7
diff --git a/tempest/common/utils/__init__.py b/tempest/common/utils/__init__.py
index 38f3d38..04d898d 100644
--- a/tempest/common/utils/__init__.py
+++ b/tempest/common/utils/__init__.py
@@ -1,4 +1,3 @@
-LAST_REBOOT_TIME_FORMAT = '%Y-%m-%d %H:%M:%S'
 PING_IPV4_COMMAND = 'ping -c 3 '
 PING_IPV6_COMMAND = 'ping6 -c 3 '
 PING_PACKET_LOSS_REGEX = '(\d{1,3})\.?\d*\% packet loss'