backend: remove a couple of useless mkdir calls

Those directories must exist for the node daemon to run (it's in the
node daemon's list of ensured directories) and those functions are only
called by the node daemon, so there's no point in those checks+mkdir
calls.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>