libstdc++: Remove more redundant union members

We don't need these 'unused' members because they're never used, and a
union with a single variant member is fine.

libstdc++-v3/ChangeLog:

	* libsupc++/eh_globals.cc (constant_init::unused): Remove.
	* src/c++11/system_error.cc (constant_init::unused): Remove.
	* src/c++17/memory_resource.cc (constant_init::unused): Remove.