SUNRPC: Remove unused declaration rpc_modcount()

These declarations are never implemented since the beginning of git
history. Remove these, then merge the two #ifdef block for
simplification.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>