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]