Remove use of non-existing buildbot envvar in retrace header.

Change-Id: Icde7cd43b569849db3f15151fc307de9fdef4956
diff --git a/build.gradle b/build.gradle
index e83113a..178d8e8 100644
--- a/build.gradle
+++ b/build.gradle
@@ -1900,10 +1900,6 @@
     def err = new StringBuffer()
     def command = "python tools/retrace.py --quiet"
     def header = "RETRACED STACKTRACE";
-    if (System.getenv('BUILDBOT_BUILDERNAME') != null
-            && !System.getenv('BUILDBOT_BUILDERNAME').endsWith("_release")) {
-        header += ": (${command} --commit_hash ${System.getenv('BUILDBOT_REVISION')})";
-    }
     out.append("\n--------------------------------------\n")
     out.append("${header}\n")
     out.append("--------------------------------------\n")