Don't use QSettings to "cache" plugin information

Don't use QSettings to "cache" plugin information

The main rationale of the cache was to examine the plugin's build-key
before loading it. Now that the build-key has been removed, the cache
has lost its usefulness.

This is part of a larger push to not use QSettings for Qt specific
settings or caches.

See also:
http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-August/000892.html
http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-August/000960.html
http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-August/000907.html
http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-August/000904.html

Change-Id: I96e84aa25983c8e06e027ff70cef109444c362a2
Reviewed-on: http://codereview.qt.nokia.com/3978
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>