Remove unused filesystem stat symbols.

Remove unused filesystem stat symbols.

d_fs_data_s	HAS_STRUCT_FS_DATA
d_fstatfs	HAS_FSTATFS
d_fstatvfs	HAS_FSTATVFS
d_getfsstat	HAS_GETFSSTAT
d_getmnt	HAS_GETMNT
d_getmntent	HAS_GETMNTENT
d_hasmntopt	HAS_HASMNTOPT
d_statfs_f_flags	HAS_STRUCT_STATFS_F_FLAGS
d_statfs_s	HAS_STRUCT_STATFS
d_ustat	HAS_USTAT
i_mntent	I_MNTENT
i_sysmount	I_SYS_MOUNT
i_sysstatfs	I_SYS_STATFS
i_sysstatvfs	I_SYS_STATVFS
i_sysvfs	I_SYS_VFS
i_ustat	I_USTAT

Unused by the Perl core.

As far as I can remember I added these scans long ago, for some
purpose (df(1) kind of APIs?) but whatever it was, it obviously
hasn't exactly caught fire in the last 15 years.

Some rare uses of these APIs (not these defines, but e.g. statfs)
in CPAN (like the Quota module), but those seem to do their own
configuration.