[2.x] Remove the group property from the navigation item class

[emmadesilva]

Targets #1568.

Do we actually need this? We should just care if it's physically stored in a group. Adding the group key here doesn't magically make it appear in the group, it needs to be added to the group's items array, so why should we care about the key here?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (c33a6ec) to head (de6993b).

Additional details and impacted files

@@                         Coverage Diff                         @@
##             improved-navigation-internals    #1639      +/-   ##
===================================================================
- Coverage                            99.93%   99.93%   -0.01%     
+ Complexity                            1779     1778       -1     
===================================================================
  Files                                  184      184              
  Lines                                 4802     4799       -3     
===================================================================
- Hits                                  4799     4796       -3     
  Misses                                   3        3              

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