[tor-bugs] #17882 [Core Tor/Tor]: Remove needless *_support_ntor()

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#17882: Remove needless *_support_ntor()
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 Reporter:  nickm                                |          Owner:  (none)
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Low                                  |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:
 Severity:  Trivial                              |     Resolution:
 Keywords:  easy tor-client code-removal         |  Actual Points:
  technical-debt                                 |
Parent ID:                                       |         Points:  small
 Reviewer:                                       |        Sponsor:
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Changes (by irl):

 * status:  new => needs_review


Comment:

 In `33da2abd05`, directory authorities began rejecting relays that did not
 publish a valid ntor key. This means that the consensus will only contain
 relays that do have ntor keys published in their descriptors.

 `circuit_cpath_supports_ntor` is indeed redundant and only duplicating the
 check that has already been performed by the directory authorities.

 There is one other function in the `_supports_ntor` realm:
 `extend_info_supports_ntor`. dir-spec currently does not require ntor in
 extra info descriptors and so this is not yet removable. For example, v2
 hidden services still rely on the TAP handshake.

 The above patch removes `circuit_cpath_supports_ntor`. `make test` passes
 and I have tested client functionality for bootstrapping, "normal"
 circuits, v2 hidden service circuits and v3 hidden service circuits. It
 also updates a comment in `dirserv.c` to explain that clients will now
 assume ntor support, where it previously said clients would check.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17882#comment:8>
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