Remove deprecated std::has_trivial_xxx traits

[Jonathan Wakely]

I propose that we remove the following non-standard traits in GCC 7:

  /// has_trivial_default_constructor (temporary legacy)
  template<typename _Tp>
    struct has_trivial_default_constructor
    : public integral_constant<bool, __has_trivial_constructor(_Tp)>
    { } _GLIBCXX_DEPRECATED;

  /// has_trivial_copy_constructor (temporary legacy)
  template<typename _Tp>
    struct has_trivial_copy_constructor
    : public integral_constant<bool, __has_trivial_copy(_Tp)>
    { } _GLIBCXX_DEPRECATED;

  /// has_trivial_copy_assign (temporary legacy)
  template<typename _Tp>
    struct has_trivial_copy_assign
    : public integral_constant<bool, __has_trivial_assign(_Tp)>
    { } _GLIBCXX_DEPRECATED;

They were in C++0x drafts but were removed before the final C++11
standard, so have never been part of ISO C++. They've been deprecated
since GCC 5.1: https://gcc.gnu.org/gcc-5/changes.html

People who need that functionality can still use the built-ins
directly, we don't need to define non-standard traits.

Alternatively, we could move them to __gnu_cxx, so they don't pollute
the namespace by default, or only define them when __STRICT_ANSI__ is
not defined. I prefer simply removing them.