Remove linux/ubi-user.h

Remove linux/ubi-user.h

As linux kernel provides a compatible mtd/ubi-user.h header since
commit v3.7-rc1~85^2~17, there is no need for a local copy.

* linux/ubi-user.h: Remove.
* Makefile.am (EXTRA_DIST): Remove it.
* configure.ac (AC_CHECK_MEMBERS): Check
for struct ubi_attach_req.max_beb_per1024 in <mtd/ubi-user.h>.
* ioctl.c (ioctl_decode) <case 'o', case 'O'>: Conditionalize
on HAVE_STRUCT_UBI_ATTACH_REQ_MAX_BEB_PER1024.
* ubi.c: Likewise.