[cleanup] Remove warning for FS#493 in sllisttest.

The warning was added before FS#493 and the related FS#491
were resolved. Probably it was just forgotten to remove the
warning.
I double checked with GCC 5 and -O3. Works for me.

dune/common/test/sllisttest.cc

@@ -4,8 +4,6 @@
 #include "config.h"
 #endif
 
-#warning sllist is known to fail with some optimization flags (FS493)
-
 #include <dune/common/sllist.hh>
 #include <dune/common/test/iteratortest.hh>
 #include <dune/common/poolallocator.hh>