ubifs: Remove ineffective function ubifs_evict_xattr_inode()

Function ubifs_evict_xattr_inode() is imported by commit 272eda82
("ubifs: Correctly evict xattr inodes") to reclaim xattr inode when
the host inode is deleted.
The xattr inode is evicted in the host inode deleting process since
commit 7959cf3a ("ubifs: journal: Handle xattrs like files").
So the ineffective function ubifs_evict_xattr_inode() can be deleted
safely.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>