_q_isSymbianHidden is doubling the cost of QFile::Exists calls

Some performance traces of QFile::Exists on Symbian show that there are two calls to RFs::Entry. The first is the one expected from doStat(), the second is from _q_isSymbianHidden() - this should virtually never be needed, as we should already know if it is a link or not and therefore not need this call.

It could be that this is of very low relevance. If the native file system changes are adopted, this code will be replaced anyway.