Do not check for struct stat.st_{a,c}time_nsec

Do not check for struct stat.st_{a,c}time_nsec

Assume that the check for struct stat.st_mtime_nsec is enough.
Likewise, do not check for struct stat.st_{a,c}tim.tv_nsec.

* configure.ac (AC_CHECK_MEMBERS): Remove struct stat.st_atime_nsec,
struct stat.st_ctime_nsec, struct stat.st_atim.tv_nsec,
and struct stat.st_ctim.tv_nsec.
* file.c (HAVE_STRUCT_STAT_ST_ATIME_NSEC,
HAVE_STRUCT_STAT_ST_CTIME_NSEC): Remove.
* printstat.h (HAVE_STRUCT_STAT_ST_ATIME_NSEC,
HAVE_STRUCT_STAT_ST_CTIME_NSEC): Replace with
HAVE_STRUCT_STAT_ST_MTIME_NSEC.
* tests/xstatx.c: Likewise.