[autotest] performance_optional only exists in ToT, do not run it in older branches

This CL can be reverted if CL 257010 is ported back to older branches.

BUG=chromium:465054
TEST=None

Change-Id: I886bc9a18741fb469c867e8626bb86d312dfaa18
Reviewed-on: https://chromium-review.googlesource.com/257211
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
diff --git a/suite_scheduler.ini b/suite_scheduler.ini
index 3eaeb64..a98e6ba 100644
--- a/suite_scheduler.ini
+++ b/suite_scheduler.ini
@@ -737,6 +737,6 @@
 [PerformanceOptional]
 run_on: nightly
 suite: performance_optional
-branch_specs: >=tot-2
+branch_specs: ==tot
 pool: performance
 file_bugs: True