Remove defunct plugin-loading code, and deprecate KLibrary

KLibrary::factory() tried to load kde3-style plugins (not unreasonable
to drop support for) and kdelibs4-style plugins (which do not work,
since they depended on Qt support). It now does nothing.

This and the KPluginFactory::findPlugin() method leave KLibrary with no
purpose, so deprecate it.

(REVIEW): 116938

Commit b779f3478b02fc60b3005ad00a7859519d70d9dc in kservice