Symbian Bearer plugin stub deployment fails on some SDKs

Symbian Bearer plugin stub deployment in s60installs.pro fails on some SDKs. This happens when SDK has epoc32/include/data_caging_paths.prf, too, and Qt's data_caging_paths.prf is overridden by that. However, the root problem is that QT_PLUGINS_BASE_DIR and QT_IMPORTS_BASE_DIR should not be platform overridable in the first place since in that case QLibraryInfo::location() calls in code will not match these paths. Optimally these variables should be removed from data_caging_paths.prf and be set in qconfig.pri which is generated by configure.