Index builds used to not support document deletion while the index was being built, which caused major problems for initial sync of capped collections (see SERVER-29197). To work around this, initial sync special cases capped collections and creates indexes on them before inserting any documents.

The root problem was fixed by hybrid index builds, and MultiIndexBlock now does the right thing if documents are deleted during an index build. This means that we can remove the special-casing and eliminate a place that index idents need to be plumbed through.