CPAlgorithm: clean up PtEtaSelection and ObjectCutflow

Remove the postfix option from the PtEtaSelectionBlock and ObjectCutFlowBlock config blocks. This option was only used to provide a unique name for the algorithms when calling the blocks several times - this can be done internally by looking up the selectionName instead.

Also remove the makePtEtaSelectionConfig and makeObjectCutFlowConfig methods: these are completely equivalent to setting up the blocks directly and were not used anywhere in the common code.