Remove scriptlets

Created by: aimylios

Even after all the cleanup we have done in the last months, the SPEC file still includes some old and now obsolete constructs. These commits remove the %post, %postun and %posttrans sections, as none of the scriptlets is required anymore:

• mimeinfo is deprecated since Fedora 24
  • https://fedoraproject.org/w/index.php?title=Packaging:Scriptlets&oldid=494555#mimeinfo
  • https://pagure.io/packaging-committee/issue/636
• desktop-database is deprecated since Fedora 25
  • https://fedoraproject.org/w/index.php?title=Packaging:Scriptlets&oldid=494574#desktop-database
• icon cache has been deprecated last year and is currently being removed from all upstream packages
  • https://pagure.io/packaging-committee/issue/736