[SPARK-28780][ML] Delete the incorrect setWeightCol method in LinearSVCModel

[zhengruifeng]

What changes were proposed in this pull request?

Delete the incorrect method def setWeightCol(value: Double): this.type = set(threshold, value) in LinearSVCModel

Why are the changes needed?

LinearSVCModel should not provide this setter, moreover, this method is wrongly defined.

Does this PR introduce any user-facing change?

yes, a public method is removed

How was this patch tested?

existing suites

[SparkQA]

Test build #109400 has finished for PR 25510 at commit dcaea26.

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

[SparkQA]

Test build #109403 has finished for PR 25510 at commit ce175ca.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[srowen]

Agree, that looks like a copy-paste error. I'm tempted to say we should make it a no-op in 2.x as well, just in case somebody is calling it and accidentally changing the threshold instead. However they'd already be hitting an error if so. We can probably just remove for 3.0.

[dongjoon-hyun]

Thank you, @zhengruifeng and @srowen .

+1, LGTM for 3.0.

Since this API is removed, we need to follow a deprecation process in 2.4.4 and 2.3.4. (cc @kiszk for 2.3.4).

1. Shall we mark a deprecation annotation on 2.4.4 and 2.3.4?
2. Make the code no-op and show warning.
3. Document it in the ML migration guide?

[dongjoon-hyun]

cc @dbtsai

[srowen]

That wouldn't hurt to 'back port' a different change that deprecates it and makes it emit a warning only. We can get it into 2.4.4, 2.3.4 indeed. I'm not super concerned about it because a call to setWeightCol("myCol") would not even compile right now. I can't see that anyone would ever have used it. (Yes I already put something in the release notes, but not sure a migration guide entry makes sense.)

[zhengruifeng]

@srowen @dongjoon-hyun I am OK to make another prs for 2.3.4 & 2.4.4.
Maybe two PRs are needed, since the version in deprecation annotation are different.
Thanks for reviewing!

[srowen]

Merged to master

[dongjoon-hyun]

Yep. Please make two PR for them, @zhengruifeng . Thanks.