Revert "[lldb] avoid assert in threadsanitizer tests on linux"

This reverts commit f369d51896. The bug this
fixes was already fixed by 1c5a0cb1c3 with the
same approach and this commit is now just giving the variable a second fallback
value.

lldb/source/Plugins/InstrumentationRuntime/TSan/InstrumentationRuntimeTSan.cpp

@@ -855,8 +855,6 @@ bool InstrumentationRuntimeTSan::NotifyBreakpointHit(
         });
     report->GetAsDictionary()->AddBooleanItem("all_addresses_are_same",
                                               all_addresses_are_same);
-  } else {
-    stop_reason_description = "unknown ThreadSanitizer stop reason";
   }
 
   // Make sure this is the right process