Remove the need for application to set an RPATH.

The WebEngineWidgets module and the WebEngine QtQuick plugin libraries
already have the RPATH set properly in their headers and the application
won't need to link any symbol directly to the Core library.

Remove the RPATH directive for examples and tests and fix the build issue
by making sure that the link directive isn't passed to dependencies
through the prl or pkgconfig file.

Change-Id: Id1f5efb8c9823613e804e8e6356d711d561d72ec
Reviewed-by: Andras Becsi <andras.becsi@digia.com>