Remove TCP_SAD optional code now that the sack filter performs this function.

[rrs]

Summary

With the commit of D44903 we no longer need the SAD option. Instead all stacks that
use the sack filter inherit its protection against sack-attaack.

Test Plan

Make sure that removal does not break anything

a) Run the TCP test suite for rack
b) Run the 4 pkt-drill scripts from D44903.