Remove support for removing glibc 2.0 headers.

There is a configure test for the presence of glibc 2.0 headers (that
were renamed / no longer installed in glibc 2.1) and associated
support for removing them on "make install".

Normal practice for subsequent removal / renaming of installed files
has been not to do anything special about removing them; if you want
installed files from an old installation removed reliably, you need to
use a packaging system that tracks what files were installed by a
previous glibc package (via installing in an intermediate directory
with install_root).  I think it's been long enough since 2.0 that it's
not particularly useful to have that special logic for those old
headers either; this patch removes it.

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).

	* configure.ac (old_glibc_headers): Remove configure test.
	* configure: Regenerated.
	* config.make.in (old-glibc-headers): Remove variable.
	* Makefile [!$(install_root) && $(old-glibc-headers) = yes]
	(install): Remove dependency on remove-old-headers.
	(headers2_0): Remove variable.
	(remove-old-headers): Remove rule.