[Bugs] #1651 UNSP: simplify jarabe.journal.model._Cache

[Sugar Labs Bugs]

#1651: simplify jarabe.journal.model._Cache
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    Reporter:  sascha_silbe               |          Owner:  sascha_silbe               
        Type:  enhancement                |         Status:  new                        
    Priority:  Unspecified by Maintainer  |      Milestone:  Unspecified by Release Team
   Component:  sugar                      |        Version:  Git as of bugdate          
    Severity:  Unspecified                |       Keywords:  r?                         
Distribution:  Unspecified                |   Status_field:  Assigned                   
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 jarabe.journal.model._Cache can be simplified a great deal just by
 changing the way removes are done. This decreases memory usage (as we
 don't need to keep a dictionary that just duplicates the list) and also
 fixes a bug I encountered in my version support branch (but couldn't
 trigger on mainline).

 I've kept the class to minimize the code changes, but I cannot see a
 reason why we'd need it - a simple list should do.

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Ticket URL: <http://bugs.sugarlabs.org/ticket/1651>
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