[cleanup] Remove backwards-compatible feature macros.

They were tested until 2.3 and are now always supported due to
the new compiler requirements.
Removed macros: HAVE_INTEGRAL_CONSTANT, HAVE_DUNE_HASH,
HAVE_STD_HASH, HAVE_TYPE_TRAITS, HAVE_VARIADIC_TEMPLATES,
HAVE_VARIADIC_CONSTRUCTOR_SFINAE, HAVE_RVALUE_REFERENCES

config.h.cmake

@@ -26,17 +26,6 @@
 /* does the compiler support __attribute__((unused))? */
 #cmakedefine HAS_ATTRIBUTE_UNUSED 1
 
-/* old feature support macros which were tested until 2.3, keep around for one more release */
-/* As these are now always supported due to the new compiler requirements, they are directly */
-/* defined without an explicit test. */
-#define HAVE_INTEGRAL_CONSTANT 1
-#define HAVE_DUNE_HASH 1
-#define HAVE_STD_HASH 1
-#define HAVE_TYPE_TRAITS 1
-#define HAVE_VARIADIC_TEMPLATES  1
-#define HAVE_VARIADIC_CONSTRUCTOR_SFINAE 1
-#define HAVE_RVALUE_REFERENCES 1
-
 /* Define if you have a BLAS library. */
 #cmakedefine HAVE_BLAS 1