SUNRPC: sunrpc should not explicitly depend on NFS config options

Change explicit references to CONFIG_NFS_V4_1 to implicit ones
Get rid of the unnecessary defines in backchannel_rqst.c and
bc_svc.c: the Makefile takes care of those dependency.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>