Remove unreliable test

[devmotion]

The motivation for the test is to check that out-of-place cov2cor and cor2cov of Hermitian and Symmetric matrices only write to the active triangle. But it's a bit unreliable (see e.g. https://github.com/JuliaStats/StatsBase.jl/actions/runs/18417193235?pr=965) as we don't have any control over how the output matrix is actually initialized in

StatsBase.jl/src/cov.jl

Line 132 in 92d1674

return cov2cor!(copyto!(similar(C, T), C), s)

and

StatsBase.jl/src/cov.jl

Line 195 in 92d1674

return cor2cov!(copyto!(similar(C, T), C), s)

.

[devmotion]

The doctest error is fixed in #966.