ceph: kill __ceph_removexattr()

when removing a xattr, generic_removexattr() calls __ceph_setxattr()
with NULL value and XATTR_REPLACE flag. __ceph_removexattr() is not
used any more.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>