Remove sizeof (long double) configure test.

There is a configure test for sizeof (long double) whose results
aren't used anywhere.  This patch removes that test.

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

	* configure.ac (sizeof_long_double): Remove configure test.
	* configure: Regenerated.
	* config.make.in (sizeof-long-double): Remove variable.