ecdsa: remove unnecessary check in FromStr impl

[newpavlov]

We already check that hex.len() == C::FieldBytesSize::USIZE * 4, so split_at should not panic and any invalid strings should be properly handled by parses.

[newpavlov]

We could harden the hex.len() check to account for potential overflow by using hex.len() % 4 != 0 || hex.len() / 4 != C::FieldBytesSize::USIZE, but I am not sure it's worth the trouble since it only matters for impractical values of C bigger than usize::MAX / 4.