Clean up where the QSettings is retrieved from.

The settings in the plugin manager (local/global) are where the plugin
manager is supposed to look for its settings, which has per se nothing
to do where Qt Creator wants other settings to be written to.
All Qt Creator code should use ICore::settings() for that instead.

Change-Id: I28bd855ae6cbcdf8d19017eb44f178e4624a1a61
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>