Remove CONFIG_UBIFS_FS_XATTR configuration option and associated
UBIFS_FS_XATTR ifdefs.

Testing:
       Tested using integck while using nandsim on x86 & MX28 based
       platform with Micron MT29F2G08ABAEAH4 nand.
Signed-off-by: NSubodh Nijsure <snijsure@grid-net.com>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>