Remove redundant rebalancing of P4estMesh

[efaulhaber]

Once cburstedde/p4est#112 is fixed, we should remove redundant calls to p4est_balance like in code like this:

Trixi.jl/src/solvers/dg_p4est/dg.jl

Lines 7 to 10 in dd5e28e

	p4est_balance(mesh.p4est, P4EST_CONNECT_FACE, C_NULL)
	# Due to a bug in p4est, the forest needs to be rebalanced twice sometimes
	# See https://github.com/cburstedde/p4est/issues/112
	p4est_balance(mesh.p4est, P4EST_CONNECT_FACE, C_NULL)

[efaulhaber]

See updates in cburstedde/p4est#112 (comment).
It appears that the way we currently set up the connectivity is not supported, and that we should also set up corner connectivity, even though we don't need that for DG.
That should make this double rebalancing redundant.

This is definitely more work on our side, but from these comments, it seems that our current approach might break one day when they start relying more heavily on corner connectivity.