Makefile.def: drop remnants of unused libelf

Use of libelf was removed from gcc in r0-104274-g48215350c24d52 ("re PR
lto/46273 (Failed to bootstrap)") around 2010, before gcc-4.6.0.

This change removes unused references to libelf from top-level configure
and Makefile.

	* Makefile.def: Drop libelf module and gcc-configure dependency
	on it.
	* Makefile.tpl (HOST_EXPORTS): Drop unused LIBELFLIBS and
	LIBELFINC.