Remove unused variables and refactor GENE_LIST

Noted in previous PRs that that the `GENES` and `GENES_SPACE_DELIMITED`
variables are not needed¹ or used in the workflow,² so refactor the
`GENE_LIST` to be a hardcoded list of genes.

If we want to ensure that we do not miss any genes from the Nextclade
dataset, we could parse out the gene names from the dataset's
genome_annotation.gff file. However, I think that will over-complicate
the Snakemake workflow so I'm leaving the hardcoded list.

¹ #372 (comment)
² #435 (comment)

Snakefile

@@ -5,9 +5,12 @@ import os
 # Snakemake 7.7.0 introduced `retries` directive used in fetch_sequences
 min_version("7.7.0")
 
-GENES = "E,M,N,ORF1a,ORF1b,ORF3a,ORF6,ORF7a,ORF7b,ORF8,ORF9b,S"
-GENES_SPACE_DELIMITED = GENES.replace(",", " ")
-GENE_LIST = GENES.split(",")
+# Hardcoded gene list used to create the DAG for both nextclade.smk and upload.smk
+# It does _not_ need to be supplied to the `nextclade run` invocations because
+# it matches the genes listed in the SARS-CoV-2 Nextclade dataset genome_annotations.gff
+# https://github.com/nextstrain/nextclade_data/blob/244058e7d599a8295d748b12cffdd25cec6d3e7b/data/nextstrain/sars-cov-2/wuhan-hu-1/orfs/genome_annotation.gff3
+#   - Jover, 21 Feb 2024
+GENE_LIST = ['E', 'M', 'N', 'ORF1a', 'ORF1b', 'ORF3a', 'ORF6', 'ORF7a', 'ORF7b', 'ORF8', 'ORF9b', 'S']
 
 #################################################################
 ####################### general setup ###########################