Revert "libstdc++: Export global iostreams with GLIBCXX_3.4.31 symver [PR108969]"

This reverts commit b7c54e3f.

libstdc++-v3/ChangeLog:

	* config/abi/post/aarch64-linux-gnu/baseline_symbols.txt:
	* config/abi/post/i486-linux-gnu/baseline_symbols.txt:
	* config/abi/post/m68k-linux-gnu/baseline_symbols.txt:
	* config/abi/post/powerpc64-linux-gnu/baseline_symbols.txt:
	* config/abi/post/riscv64-linux-gnu/baseline_symbols.txt:
	* config/abi/post/s390x-linux-gnu/baseline_symbols.txt:
	* config/abi/post/x86_64-linux-gnu/32/baseline_symbols.txt:
	* config/abi/post/x86_64-linux-gnu/baseline_symbols.txt:
	* config/abi/pre/gnu.ver:
	* src/Makefile.am:
	* src/Makefile.in:
	* src/c++98/Makefile.am:
	* src/c++98/Makefile.in:
	* src/c++98/globals_io.cc (defined):
	(_GLIBCXX_IO_GLOBAL):