Remove TLS configure tests.

There seemed to be support in response to
<https://sourceware.org/ml/libc-alpha/2015-10/msg00510.html> for
removing configure tests that exist only to produce errors, where we
expect that tool versions failing the tests would also fail the tests
of minimum GCC / binutils versions.

This patch removes the tests for TLS support as one instance of such
tests.  Since the addition of TLS emulation support in GCC 4.3, I
don't think these tests would have failed even if proper TLS support
(as required by glibc) was missing in that architecture's GCC back
end, so any new glibc ports wanting a substantive test (if there are
actual GCC or binutils versions for those architectures, with
supported version numbers, missing TLS support) would have needed an
architecture-specific test anyway.

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

	* configure.ac (libc_cv_gcc___thread): Remove configure test.
	(libc_cv_gcc_tls_model_attr): Likewise.
	* configure: Regenerated.