SL: move fake line prescales from Spruce to HLT2

Andy Morrisrequested to merge
anmorris-hlt2-fakeLepton-prescale into master

The prescaling used to reduce the bandwidth on SLB background lines at the sprucing stage should instead be applied to the HLT2 stage.

The prescales at the Sprucing stage are modified to compensate for this.

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