NFS: Remove CONFIG_NFS_V4 checks from nfs_idmap.h

The idmapper is completely internal to the NFS v4 module, so this macro
will always evaluate to true.  This patch also removes unnecessary
includes of this file from the generic NFS client.

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