Checkpoint workspace iterator removal.

Refs #8983
Remove WorkspaceIterator from SaveRKH.

Refs #8983
Remove workspace iterators from UserAlgorithms

Refs #8983
Remove WorkspaceIterator code

Refs #8983
Remove WorkspaceIteratorCodeTest

Refs #8983
Remove WorkspaceIterator usage in tests.

Refs #8983
Merge branch 'task/8983_rm_workspaceitr' into develop