GH-4999: improve UpdateWithModelBuilder to support statement removal

[fkleedorfer]

GitHub issue resolved: #4999

Briefly describe the changes proposed in this PR:

The UpdateWithModelBuilder used to be just an Operation with an internal ModelBuilder that is exposed to clients. thus, one can only add statements with it, but not delete any. In practice, it turns out that sometimes, the data for one bigger update is often collected iteratively, which can lead to the need for removing some of the statements already added.

With this update, the UpdateWithModel builder allows for removal of statements, and applies additions and removals correctly when its execute() method is called.

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