Remove bitrotten --enable-oldest-abi (bug 6652).

This patch removes the --enable-oldest-abi configure option, which has
long been bitrotten (as reported in bug 6652).  The principle of
removing this option was agreed in the thread starting at
<https://sourceware.org/ml/libc-alpha/2013-07/msg00174.html>.

Tested for x86_64 and x86 that the installed shared libraries other
than libc.so are unchanged by this patch and that libc.so disassembly
and symbol versions are unchanged (debug info changes because of
changed line numbers in csu/version.c).

	[BZ #6652]
	* Makeconfig (soversions-default-setname): Remove variable.
	($(common-objpfx)soversions.i): Don't pass default_setname to
	soversions.awk.
	* Makerules ($(common-objpfx)abi-versions.h): Don't pass
	oldest_abi to abi-versions.awk.
	* config.h.in (GLIBC_OLDEST_ABI): Remove macro undefine.
	* config.make.in (oldest-abi): Remove variable.
	* configure.ac (--enable-oldest-abi): Remove configure option.
	* configure: Regenerated.
	* csu/version.c (banner) [GLIBC_OLDEST_ABI]: Remove conditional
	text.
	* scripts/abi-versions.awk: Do not handle oldest_abi variable.
	* scripts/soversions.awk: Do not handle default_setname variable.
	* sysdeps/mach/hurd/configure.ac: Do not handle oldest_abi
	variable.
	* sysdeps/mach/hurd/configure: Regenerated.
	* sysdeps/unix/sysv/linux/configure.ac: Do not handle oldest_abi
	variable.
	* sysdeps/unix/sysv/linux/configure: Regenerated.

scripts/soversions.awk

@@ -27,7 +27,7 @@ END {
     if (lib in versions)
       set = versions[lib];
     else {
-      set = default_setname;
+      set = "";
       if (ndefault_set >= 1)
 	set = default_set[1];
     }