chore: remove temporary decide simp lemmas now subsumed by core

[attilavjda]

Issue #, if available:

Description of changes:

Cedar/Thm/Data/LT.lean defines four @[simp] lemmas in namespace Decide, with the comment

"Temporary simp lemmas. We should be able to get rid of these when we upgrade to a future version of Lean."

Batteries simpNF linter (lake exe runLinter Cedar) reports all four as redundant,

core simp proves each via decide_eq_true_eq, decide_eq_false_iff_not, decide_not, and Bool.not_eq_eq_eq_not,

no call sites, neither name is referenced anywhere,

lake build Cedar SymCC UnitTest SymTest is green with zero warnings.

Used Aristotle AI (Harmonic) in making this PR.