Remove all-element-non-negative check from user-prior CMA-ES

As this check is not necessary (and even malicious), I remove this
check. I believe this check was simply forgotten to be removed.

package/samplers/user_prior_cmaes/sampler.py

@@ -91,8 +91,6 @@ def _validate_user_prior(
             )
         if not np.allclose(cov0, cov0.T):
             raise ValueError("cov0 must be a symmetric matrix.")
-        if np.any(cov0 < 0.0):
-            raise ValueError("All elements in cov0 must be non-negative.")
         if np.any(np.linalg.eigvals(cov0) < 0.0):
             raise ValueError("cov0 must be a semi-positive definite matrix.")