Remove ISOLATESERVER from lib/constants.py

Isolated server will be shutdown. and this constant doesn't look used.

BUG=chromium:1163817
TEST=None

Change-Id: Ia62f2f5f2b494e3ec459ebf92b40f122b3f22f50
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2626793
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@google.com>
Tested-by: Prathmesh Prabhu <pprabhu@google.com>
diff --git a/lib/constants.py b/lib/constants.py
index 6743e30..dfe5b88 100644
--- a/lib/constants.py
+++ b/lib/constants.py
@@ -230,8 +230,6 @@
 # buildbot generation) is a unique identifier of builds.
 BUILDBOT_GENERATION = 1
 
-ISOLATESERVER = 'https://isolateserver.appspot.com'
-
 GOOGLE_EMAIL = '@google.com'
 CHROMIUM_EMAIL = '@chromium.org'