genDeleteRows is mostly running on non-existing keys

[dkropachev]

https://github.com/scylladb/gemini/blob/c93899e0bdc8993ce3c0a3d50be54df1b7eaf70b/pkg/jobs/gen_mutate_stmt.go#L31-L61

Row deletion should be running on both existing and non-existing records with different ration. We need to split them into different statements type.