[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.)
issue