BIND 10 #639: remove empty code in lib/cache

[BIND 10 Development]

#639: remove empty code in lib/cache
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           Reporter:  jinmei         |                      Owner:
               Type:  defect         |                     Status:  new
           Priority:  minor          |                  Milestone:  A-Team-
          Component:  resolver       |  Task-Backlog
          Sensitive:  0              |                   Keywords:
Add Hours to Ticket:  0              |  Estimated Number of Hours:  0
        Total Hours:  0              |                  Billable?:  1
                                     |                  Internal?:  0
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 See https://lists.isc.org/pipermail/bind10-dev/2011-February/002037.html
 for the background issue.

 The proposed action has been agreed at the biweekly call on Mar 1.

 Specific task actions are:
  - remove MessageCache/RRsetCache/ResolverCache::dump/load/resize
  - add comments about future possibility of these features
  - remove the corresponding suppression rule in src/cppcheck-suppress.lst

 This is mostly a trivial task, but I'm creating a ticket explicitly
 as I proposed in the call.  This is a minor cleanup but I'd like to
 do it before the end of y2.

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Ticket URL: <https://bind10.isc.org/ticket/639>
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