src/multi_open_unlink: Stop using attr_set

src/multi_open_unlink: Stop using attr_set

Avoid the following warning by replacing deprecated
attr_set with setxattr:
---------------------------------------------------
warning: 'attr_set' is deprecated: Use setxattr or lsetxattr instead [-Wdeprecated-declarations]
---------------------------------------------------

Also remove unneeded flags, <attr/attributes.h> and $(LIBATTR).

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>