Remove discrete derivative material

It won't be used in any combinatorics course nor my PhD, hence doesn't seem like a good fit for this project. Furthermore, Mathlib recently acquired `fwdDiff`, which is a very similar definition to `discConv`, and someone provided Lean code on Zulip that proves the original theorem that motivated this file. See https://leanprover.zulipchat.com/#narrow/channel/287929-mathlib4/topic/Supplement.20to.20the.20fwdDiff.20theorem.20in.20mathelib.

Author: YaelDillies

LeanCamCombi.lean

@@ -3,7 +3,6 @@ import LeanCamCombi.CauchyFunctionalEquation
 import LeanCamCombi.ConvexityRefactor.Defs
 import LeanCamCombi.ConvexityRefactor.StdSimplex
 import LeanCamCombi.Corners.CombiDegen
-import LeanCamCombi.DiscreteDeriv
 import LeanCamCombi.ExtrProbCombi.BernoulliSeq
 import LeanCamCombi.ExtrProbCombi.BinomialRandomGraph
 import LeanCamCombi.ExtrProbCombi.BollobasContainment