NFSD: Remove unused variable from nfsd4_decode_bind_conn_to_session()

Compiling gave me this warning:
fs/nfsd/nfs4xdr.c: In function ‘nfsd4_decode_bind_conn_to_session’:
fs/nfsd/nfs4xdr.c:427:6: warning: variable ‘dummy’ set but not used
[-Wunused-but-set-variable]

The local variable "dummy" wasn't being used past the READ32() macro that
set it.  READ_BUF() should ensure that the xdr buffer is pushed past the
data read into dummy already, so nothing needs to be read in.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
[bfields@redhat.com: minor comment fixup.]
Signed-off-by: J. Bruce Fields <bfields@redhat.com>