Remove start codon check

[Koeng101]

https://github.com/TimothyStiles/poly/blob/f76bf05497cb89d5a48ab0a2ac41db9cfc425d12/synthesis/codon/codon.go#L244-L253

There should be no check here. A protein can totally be translated without a start codon - this is common if you want to translate certain portions of a protein.

[TimothyStiles]

patched in #414