mptcp: remove unused 'remaining' variable

In some functions, 'remaining' variable was given in argument and/or set
but never read.

  net/mptcp/options.c:779:3: warning: Value stored to 'remaining' is never
  read [clang-analyzer-deadcode.DeadStores].

  net/mptcp/options.c:547:3: warning: Value stored to 'remaining' is never
  read [clang-analyzer-deadcode.DeadStores].

The issue has been reported internally by Alibaba CI.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Suggested-by: Mat Martineau <martineau@kernel.org>
Co-developed-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>