[SPARK-5540] hide ALS.solveLeastSquares

[mengxr]

This method survived the code review and it has been there since v1.1.0. It exposes jblas types. Let's remove it from the public API. I think no one calls it directly.

[SparkQA]

Test build #26557 has started for PR 4318 at commit 586ade6.

• This patch merges cleanly.

[srowen]

Yes, I think it can just be hidden. This is unlikely to be called by anyone, and a quick search of github didn't turn up any obvious callers.

[SparkQA]

Test build #26557 has finished for PR 4318 at commit 586ade6.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/26557/
Test FAILed.

[mengxr]

A Spark SQL test failed. Already pinged @liancheng to take a look. I'm going to merge this.