[prop250] Remove unnecessary assertion in sr_compute_srv
In dgoulet's srv-testing-2, git 0234595:
In sr_compute_srv, this doesn't make much sense:
uint64_t reveal_num; ... tor_assert(reveal_num < UINT64_MAX);
Why stop reveal_num being equal to UINT64_MAX? There's absolutely no harm in it being equal.
Let's just delete the condition. (Or provide a comment saying why we're checking.)