Remove feature macro for <experimental/optional>

libc++ 3.8 (and possibly others--including the derived version on OS X),
doesn't define the macro, but does support std::experimental::optional.
This removes the extra macro check and just assumes the header existing
is enough, which is what we do for <optional> and <variant>.

include/pybind11/stl.h

@@ -32,10 +32,8 @@
 // std::experimental::optional (but not allowed in c++11 mode)
 #  if defined(PYBIND11_CPP14) && __has_include(<experimental/optional>)
 #    include <experimental/optional>
-#    if __cpp_lib_experimental_optional  // just in case
 #    define PYBIND11_HAS_EXP_OPTIONAL 1
 #  endif
-#  endif
 // std::variant
 #  if defined(PYBIND11_CPP17) && __has_include(<variant>)
 #    include <variant>