Remove initial cache definition

Author: albertomercurio

src/spectrum.jl

@@ -125,7 +125,6 @@ function _spectrum(
     _tr = SparseVector(D^2, [1 + n * (D + 1) for n in 0:(D-1)], ones(_CType(L), D)) # same as vec(system_identity_matrix)
     _tr_A = transpose(_tr) * spre(A).data
 
-    cache = nothing
     Id = I(D^2)
 
     # DO the idx = 1 case