Move mimetype definitions to plugin specs

- Avoids the hassle of QRC files and manually registering mime types
- Avoids performance regressions because of mime types that are
  registered after mime database has been used
- Makes it technically possible to detect that a disabled plugin could
  handle a mime type if it was enabled

Change-Id: I373008b1b56e9c6b4853055f20b3eeb112a6eff9
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cpptools/cpptools.qrc

@@ -1,6 +1,5 @@
 <RCC>
     <qresource prefix="/cpptools">
         <file>images/category_cpp.png</file>
-        <file>CppTools.mimetypes.xml</file>
     </qresource>
 </RCC>