Use dpkg triggers instead of manual update-desktop-database/update-mime-database calls

[legoktm]

Description

https://wiki.debian.org/DpkgTriggers are a more declarative way to have certain post-processing run after package install. Both update-desktop-database and update-mime-database have supported triggers since way before bullseye.

In theory we'd be able to get rid of all of:

user@dev ~/g/f/securedrop-client (main)> rg 'update-(desktop|mime)-database'
debian/securedrop-client.postinst
24:    update-desktop-database /usr/share/applications
29:    update-desktop-database /usr/share/applications

debian/securedrop-export.postinst
25:    update-desktop-database /usr/share/applications
26:    update-mime-database /usr/share/mime
36:    update-desktop-database /usr/share/applications
37:    update-mime-database /usr/share/mime

debian/securedrop-export.postrm
27:    if [[ -x "/usr/bin/update-desktop-database" ]]; then
28:        update-desktop-database /usr/share/applications
30:    if [[ -x "/usr/bin/update-mime-database" ]]; then
31:        update-mime-database /usr/share/mime