Remove useless assertion in net.nim

[Fuyukai]

This doesn't seem to be required; it only causes errors.

Fixes #4406

[Araq]

ping @dom96

[dom96]

There is a reason that assert is there, fixing that reason isn't trivial: https://github.com/nim-lang/Nim/pull/4565/files#diff-df7c4ae903eab01bc8c4f64f425b48c3R374. IIRC the problem is that the PSK client callback doesn't have access to the underlying SSL context and giving it access is also not so easy.

[dom96]

Fixed via f44486c