fix crash when more than one instances of ExtractorCollection are destructed

Summary: fix crash when more than one instances of ExtractorCollection are destructed

Test Plan:
without the modification to Extractor class, the new test fails. With the fix, valgrind does not report any new memory leak.

Due to the way QPluginLoader::instance works, the plugins are implicitly shared. If the first ExtractorCollection delete them, the second one will fail due to double delete.

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure, anthonyfieroni

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7750