SBE close() is idempotent and does not need to guard on something being open. SBE stage members are all RAII types so they will be destructed when an exception is thrown even if close() isn't called.

Reverts SERVER-124432 and SERVER-126199.

This also removes the guarding in HashAggStage. FilterStage and BranchStage maintain conditional closes because they have additional uses for the _childOpened boolean.