Remove automatic column addition for empty scan

Originally a column had to be selected to scan a table, so we always added a "cheap" column to scan.  When we removed this restriction we forgot to remove the code that automatically added a column to an empty table scan. For the Raptor connector, we must still choose a "cheap" column, but this is an internal detail of the Raptor connector.
Or rather than copying the assumption here, could do this instead

```java
for (int channel = 0; channel < types.size(); channel++) {
    if (channel == sampleWeightChannel) {
        continue;
    }