libstdc++: Remove undeclared macros from configure.ac [PR122322]

The additions in r16-4443-g651bf5126da124 cause errors when running
autoreconf.

libstdc++-v3/ChangeLog:

	PR libstdc++/122322
	* configure.ac (with_newlib) <*-rtems*>: Remove
	HAVE_SYS_IOCT4YL_H, _GLIBCXX_USE_LINK, _GLIBCXX_USE_READLINK,
	_GLIBCXX_USE_SYMLINK, _GLIBCXX_USE_TRUNCATE, and
	_GLIBCXX_USE_FDOPENDIR. Remove duplicates.
	* configure: Regenerate.

(cherry picked from commit f5fb192c65e618508108525252b771956bdb1078)