Remove FirstSamplerIncompleteSlot DB variable since it is not used

[tbenr]

setFirstSamplerIncompleteSlot and getFirstSamplerIncompleteSlot methods with the underlining DB var can be removed.

[rolfyone]

looks like they're called in DasSamplerBasic.onNewFinalizedCheckpoint

[tbenr]

yes but there is no logic on top of that value. My #10006 will remove that