CXX-322 Remove dropDups support

Perhaps we want to tag this with __attribute__((deprecated))? Not sure...

If we do so, we should do it the "right" way:
```
IndexSpec& dropDuplicates(...) ATTR_DEPRECATED {
    return dropDuplicatesDeprecated(...);
}

IndexSpec& dropDuplicatesDeprecated(...) {
   /// real code here
}
```

The advantage to this is that this way people will get the warning if they are using it, but can silence the warning even if they are using -Werror by using version that has "deprecated" in the name.