libctf: remove unnecessary zstd constructs

This patch is essentially a revert of
commit-id: 8818c80c
(libctf: Add ZSTD_LIBS to LIBS so that ac_cv_libctf_bfd_elf can be true)

As the specific configure check now uses libtool, this explicit mention of the
dependency $ZSTD_LIBS is not needed anymore.

ChangeLog:

	* libctf/Makefile.in: Regenerated.
	* libctf/aclocal.m4: Likewise.
	* libctf/config.h.in: Likewise.
	* libctf/configure: Likewise.
	* libctf/configure.ac: Remove ZSTD_LIBS from LIBS.  Cleanup
	unused AC_ZSTD.