copy: revert recent patch for vulnerable dirs

copy: revert recent patch for vulnerable dirs

I plan to propose a better patch to catch vulnerable parent
directories.
* NEWS, doc/coreutils.texi (Target directory): Document this.
* src/cp.c, src/install.c, src/ln.c, src/mv.c:
Do not include targetdir.h.
(target_directory_operand): Remove test for vulnerable parents.
* src/cp.c (stat_target_operand): Remove.  All uses removed.
* src/local.mk (noinst_HEADERS): Remove src/targetdir.h.
(src_ginstall_SOURCES, src_cp_SOURCES, src_ln_SOURCES)
(src_mv_SOURCES): Remove src/targetdir.c.
* src/targetdir.c, src/targetdir.h: Remove.
* tests/mv/vulnerable-target.sh: Remove.
* tests/local.mk (all_root_tests): Remove it.