[Scheduler] Remove superfluous m_extractorIdle variable

Summary:
Having the variable is quite pointless, as hardly anyone runs baloo
with compiled with Q_ASSERT enabled.

Test Plan:
make
no functional change

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16263