RFR: 8303410: Remove ContentSigner APIs and jarsigner -altsigner and -altsignerpath options [v3]

[Eirik Bjorsnos]

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> The `-altsigner` and `-altsignerpath` options in JarSigner with the underlying `ContentSigner` mechanism were deprected in Java 9, for removal in Java 15. See [JDK-8076535](https://bugs.openjdk.org/browse/JDK-8076535), [JDK-8242260](https://bugs.openjdk.org/browse/JDK-8242260).
> 
> This PR suggests it's time to remove this code:
> 
> - The package `com/sun/jarsigner` is removed. This contained the `ContentSigner` and `ContentSignerParameters` along with a `package-info.java` file.
> - `JarSigner.java` is updated to remove processing of the `-altsigner` and `-altsignerpath` options and the loading and processing of the custom `ContentSigner`.
> - Similarly `c.s.s.t.jarsigner.Main` is updated to remove processing and mentioning of `-altsigner` and `-altsignerpath` options.
> - Mentions of the options in Resource files in the same directory is removed
> - The `jarsigner.1` man page is updated to remove the section on the deprecated options
> - The `Spec` and `Options` tests are update to remove usage of the `-altsigner` and `-altsignerpath` options.

Eirik Bjorsnos has updated the pull request incrementally with one additional commit since the last revision:

  Remove references to com.sun.jarsigner from module-info.java and TEST.groups

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12791/files
  - new: https://git.openjdk.org/jdk/pull/12791/files/fdbcf021..9051a550

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12791&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12791&range=01-02

  Stats: 4 lines in 2 files changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12791.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12791/head:pull/12791

PR: https://git.openjdk.org/jdk/pull/12791