libstdc++: Remove unnecessary variant member in std::expected

Hui Xie pointed out that we don't need a dummy member in the union,
because all constructors always initialize either _M_val or _M_unex.

We still need the _M_void member of the expected<void, E>
specialization, because the constructor has to initialize something when
not using the _M_unex member.

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::_M_invalid): Remove.