sctp: remove old and unused SCTP_MIN_PMTU

sctp: remove old and unused SCTP_MIN_PMTU

This value is not used anywhere in the code. In essence it is a
duplicate of SCTP_DEFAULT_MINSEGMENT, which is used by the stack.

SCTP_MIN_PMTU value makes more sense, but we should not change to it now
as it would risk breaking applications.

So this patch removes SCTP_MIN_PMTU and adjust the comment above it.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>