[Stream Context] Remove staging.table.stagingSchemaName setting

[george-zubrienko]

The staging.table.stagingSchemaName is identical to staging.icebergCatalog.namespace - should replace all usage and remove the latter.

[george-zubrienko]

Moving this to 2.3 as currently this is only used for a backfill table name - will be removed as a part of backfill process re-write