[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.)