Fix postrm in deb

[LyzardKing]

An error in the postrm file could prevent reinstalls

Remove the associations file and set in .desktop
(Avoid having another extra file for a single line declaration)

Manually set license/copyright for deb (rpm in gradle):
(The parsing of the license.md file is not working consistently)

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[tobiasdiez]

Thanks. Could you please report these problem also to the jpackage devs.

[LyzardKing]

The postrm error was my fault, the copyright issue is reported upstream regarding the fact that it doesn't copy to the DEBIAN folder, I'll follow up with another email asking about the format of the license file, but I haven't heard back yet.
https://mail.openjdk.java.net/pipermail/core-libs-dev/2019-October/062905.html

[Siedlerchr]

@LyzardKing Just a reminder, you can directly create branches at the jabref upstream repo. You have push rights. So no need to work in your fork ;)