use KSycoca for updating OpenPreferredSearchTool action

Summary:
Remove all random updates of OpenPreferredSearchTool action, and
use KSycoca for updating it as discussed in https://phabricator.kde.org/D22594#663847

Enhancement for D29441 for bug #420911 caused by D22594.
The bug was fixed by D29442.

CCBUG: 420911

Test Plan:
```
$ cd ${KDE_INSTALL_DIR}/usr/share/applications

$ sudo mv org.kde.kfind.desktop org.kde.kfind.desktop_
$ kbuildsycoca5

$ sudo mv org.kde.kfind.desktop_ org.kde.kfind.desktop
$ kbuildsycoca5
```

Reviewers: broulik, elvisangelaccio, ngraham, #dolphin

Reviewed By: elvisangelaccio, #dolphin

Subscribers: anthonyfieroni, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D29568