Remove unnecessary conversions

[matiasinsaurralde]

Removes a few unnecessary conversions reported by unconvert:

% golangci-lint run -E unconvert ./...
io/gff/gff.go:253:26: unnecessary conversion (unconvert)
        featureStrand := string(feature.Strand)
                               ^
io/genbank/genbank.go:748:30: unnecessary conversion (unconvert)
    baseSequenceLength := string(basePairRegex.FindString(locusString))
                                ^
synthesis/codon/codon.go:426:27: unnecessary conversion (unconvert)
    _ = json.Unmarshal([]byte(file), &codonTable)

[gitpod-io[bot]]