Database.getDatabaseDialect's "cb" argument is never used

[odeke-em]

If we examine the method "getDatabaseDialect" https://github.com/googleapis/nodejs-spanner/blob/30151cad2723d6a9f2c70342c81f1a578ee02f6a/src/database.ts#L1530-L1553 we can see that the callback passed in is never used. I found this while working on trace instrumentation for these libraries.