RFR: 8303410: Remove ContentSigner

[Eirik Bjorsnos]

The `-altsigner` and `-altsignerpath` options in JarSigner with the underlying `ContentSigner` mechanism were deprected in Java 9, for removal in Java 16. 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.

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Commit messages:
 - Remove the com.sun.jarsigner package, the -altsigner and -altsignerpath options which have been deprecated since 9 and deprecated for removal since 16.

Changes: https://git.openjdk.org/jdk/pull/12791/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12791&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303410
  Stats: 688 lines in 13 files changed: 3 ins; 661 del; 24 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