Don't assign HSDir flag to relay that can't handle BEGIN_DIR

Child ticket of #15801 (moved).

Because of #14202 (moved) (introduced in 0.2.6), directory authorities can assign the HSDir flag to a relay without a DirPort but relay can't handle BEGIN_DIR cell if they don't have the DirPort set (see #15849 (moved)).

Commit 80bed1ac96a3035f8c55ddced5528f0d7d16d386 should be reverted and backported. Right now, we have 8 directory authorities that are >= 0.2.6 so we should backport this fix asap to 026.

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