NFS: Remove the "NFS_CAP_CHANGE_ATTR" capability

Setting the change attribute has been mandatory for all NFS versions, since
commit 3a1556e8 ("NFSv2/v3: Simulate the change attribute"). We should
therefore not have anything be conditional on it being set/unset.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>