libstdc++: Remove redundant -std flags from Makefile

In r14-4060-gc4baeaecbbf7d0 I moved some files from src/c++98 to
src/c++11 but I didn't remove the redundant -std=gnu++11 flags for those
files. The flags aren't needed now, because AM_CXXFLAGS for that
directory already uses -std=gnu++11. This removes them.

libstdc++-v3/ChangeLog:

	* src/c++11/Makefile.am: Remove redundant -std=gnu++11 flags.
	* src/c++11/Makefile.in: Regenerate.