Always delete the .sav file upon successful write

[koppor]

JabRef 5.10--2023-02-08--d708596
Windows 10 10.0 amd64 
Java 19.0.2 
JavaFX 19+11

JabRef did not delete the .sav file. RLY?

• Option 1: When starting JabRef, it should delete obsolete .sav files - or check if there is a diff in the .bib and the .sav
• Option 2: Delete .sav upon successful write, because we have "real" backups (.bak) in place - with a warning if the .bak differs from the .bib

I opted for Option 2.

Refs #9488 (comment)

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.