Remove vestigial _zerocorr while code cleaning

src/linearmixedmodel.jl

@@ -1177,45 +1177,3 @@ function StatsBase.weights(m::LinearMixedModel)
     rtwts = m.sqrtwts
     return isempty(rtwts) ? ones(eltype(rtwts), nobs(m)) : abs2.(rtwts)
 end
-
-"""
-    _zerocorr!(m::LinearMixedModel[, trmnms::Vector{Symbol}])
-
-Rewrite the random effects specification for the grouping factors in `trmnms` to zero correlation parameter.
-
-The default for `trmnms` is all the names of random-effects terms.
-
-A random effects term is in the zero correlation parameter configuration when the off-diagonal elements of
-λ are all zero - hence there are no correlation parameters in that term being estimated.
-
-Note that this is numerically equivalent to specifying a formula with `zerocorr` around each random effects
-term, but the `formula`  fields in the resulting model will differ. In particular, `zerocorr!` will **not**
-change the original `formula`'s terms to be of type of `ZeroCorr` because this would involve changing
-immutable types.  This may have implications for software that manipulates the formula of a fitted model.
-
-This is an internal function and may disappear in a future release without being considered a breaking change.
-"""
-function _zerocorr!(m::LinearMixedModel{T}, trmns) where {T}
-    reterms = m.reterms
-    for trm in reterms
-        if fname(trm) in trmns
-            zerocorr!(trm)
-        end
-    end
-    newparmap = mkparmap(reterms)
-    copyto!(m.parmap, newparmap)
-    resize!(m.parmap, length(newparmap))
-    optsum = m.optsum
-    optsum.lowerbd = mapfoldl(lowerbd, vcat, reterms)
-    optsum.initial = mapfoldl(getθ, vcat, reterms)
-    optsum.final = copy(optsum.initial)
-    optsum.xtol_abs = fill!(copy(optsum.initial), 1.0e-10)
-    optsum.initial_step = T[]
-
-    # the model is no longer fitted
-    optsum.feval = -1
-
-    return m
-end
-
-_zerocorr!(m::LinearMixedModel) = _zerocorr!(m, fnames(m))