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 https://github.com/JuliaStats/StatsBase.jl/blob/92d1674e6695dc177fc2630d1b6443b2ccf141e1/src/cov.jl#L132 and https://github.com/JuliaStats/StatsBase.jl/blob/92d1674e6695dc177fc2630d1b6443b2ccf141e1/src/cov.jl#L195.

[devmotion]

The doctest error is fixed in #966.