Revert "Backport atomic from pynest-ng to avoid data race" because it introduces a new data race.

This reverts commit c4ed2c4.

Author: heplesser

sli/token.h

@@ -359,15 +359,7 @@ class Token
   Datum*
   datum() const
   {
-    if ( not accessed_ )
-    {
-      // The if() above avoids any unnecessary updates, atomic prevents any potential
-      // data races in case the compiler does behind-the-scences magic.
-      // Backport from pynest-ng.
-#pragma omp atomic write
-      accessed_ = true;
-    }
-
+    accessed_ = true;
     return p;
   }