kontact: simplify ksycoca-related startup code

Summary:
ksycoca is rebuilt in process these days, looping and waiting
[for kbuildsycoca] makes no sense anymore [for a long time now].

CCBUG: 414137

Test Plan:
deleted ksycoca file, started kontact, it said "rebuilding ksycoca"
and started up fine

Reviewers: mlaurent, winterz, vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D25531