RFR 8242260: Remove customizable ContentSigner from jarsigner

[Weijun Wang]

After some discussion, we decide to keep the classes in JDK 15 but add a `forRemoval=true` argument. Related jarsigner help screen and warning message are also updated.

Please review everything updated at:

  Release note : https://bugs.openjdk.java.net/browse/JDK-8242261
           CSR : https://bugs.openjdk.java.net/browse/JDK-8242262
        webrev : http://cr.openjdk.java.net/~weijun/8242260/webrev.01/ (a new URL!)

Thanks,
Max

> On Apr 7, 2020, at 4:04 PM, Weijun Wang <weijun.wang at oracle.com> wrote:
> 
> I am thinking about removing the `jarsigner -altsigner -altsignerpath` options and underlying classes:
> 
>            JBS : https://bugs.openjdk.java.net/browse/JDK-8242260
> 
> Please review everything at:
> 
>   Release note : https://bugs.openjdk.java.net/browse/JDK-8242261
>            CSR : https://bugs.openjdk.java.net/browse/JDK-8242262
>         webrev : http://cr.openjdk.java.net/~weijun/8242260/webrev.00/
> 
> The CSR "Problem" section has more info on why it's better to remove it now.
> 
> Thanks,
> Max
>