We still use the IndexBuilder to build indexes in the foreground thread in applyOps. This is currently only used when creating indexes with applyOps on a primary or when rebuilding indexes during replication recovery.

We have too many duplicated implementations of index building and removing this class entirely would help to prevent diverging logic.

The functionality can be entirely replaced by using the IndexBuildsCoordinator and waiting for the future, similar to what we do in create_indexes.cpp.