krb5_child: Simplify ccache creation
The containing ccache directory is precreated by the parent code,
so there is no special need to do so here for any type.
Also the special handling for the FILE ccache temporary file is not really
useful, because libkrb5 internally unlinks and then recreate the file, so
mkstemp cannot really prevent subtle races, it can only make sure the file is
unique at creation time.