[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