Remove unused explicit instantiation of __bind_simple

The explicit instantiation of std::call_once used to require an
instantiation of __bind_simple, but call_once was changed by r241031 to
not use __bind_simple. The instantiation of __bind_simple (and the
definitions it uses) are not needed. They should have been removed
instead of doing the changes in r241111 that kept them compiling.

The use of std::call_once by _Async_state_common::_M_join can be
simplified to use a pointer instead of reference wrapper. The call_once
symbol isn't exported so the change isn't visible outside the library.

	* src/c++11/compatibility-thread-c++0x.cc [_GLIBCXX_SHARED]
	(_Async_state_common::_M_join): Simplify use of std::call_once and
	corresponding explicit instantiation.
	(_Maybe_wrap_member_pointer, _Bind_simple, _Bind_simple_helper)
	(__bind_simple): Remove definitions and explicit instantiation that
	are not required by exported symbols.

From-SVN: r262823