Remove #define made redundant by r259281

The definition of __throw_ios_failure is no longer in this file, so
setting the macro here is unnecessary.

	* src/c++11/ios.cc: Remove redundant macro definition.

From-SVN: r259349

libstdc++-v3/ChangeLog

+2018-04-12  Jonathan Wakely  <jwakely@redhat.com>
+
+	* src/c++11/ios.cc: Remove redundant macro definition.
+
 2018-04-11  Jonathan Wakely  <jwakely@redhat.com>
 
 	* doc/xml/manual/abi.xml: Document header locations in recent

libstdc++-v3/src/c++11/ios.cc

@@ -26,10 +26,6 @@
 // ISO C++ 14882: 27.4  Iostreams base classes
 //
 
-// Determines the version of ios_base::failure thrown by __throw_ios_failure.
-// If !_GLIBCXX_USE_DUAL_ABI this will get undefined automatically.
-#define _GLIBCXX_USE_CXX11_ABI 1
-
 #include <ios>
 #include <limits>