Revert "Swift: auto-flush logs at exit"

This reverts commit 0d9dcb161f.

This turns out to introduce a subtle bug related to destruction order
between `Log::instance()` and the `Logger` instances.

swift/logging/SwiftLogging.h

@@ -110,8 +110,6 @@ class Log {
     Level level;
   };
 
-  ~Log() { flushImpl(); }
-
   // Flush logs to the designated outputs
   static void flush() { instance().flushImpl(); }