[krun] Drop KToolInvocation support from KRun::runService

Summary:
This drops the use of kinit as per T12140. Few applications link to
kdeinit correctly and the potential savings that come from library
linking are now negligible.

We use the internal app spawning path for the majority of application
launches anyway via the relatively new KRun::runApplication. There isn't
a need for runService to support a more complex addition path.

It will also simplify any future changes coming to KRun.

Test Plan:
Opened a file from kicker's recently used selection (which uses this
runService)
It opened as normal, with correct startup notifications

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: ervin, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27451