Fix Radius Outlier Removal Openmp predetermined shared variables.

[larshg]

Const values are predetermined shared, gives compiler error on some GCC versions.

Fixes #6194

[mvieth]

It might be necessary to also remove default(none) then, see #6107
Alternatively, we could remove const from mean_k and nn_dists_max, and keep them in the shared clause.

[larshg]

I choose to remove the default(none) - I would prefer too keep the variables const and then lets keep the same style with openmp clauses.