[5.x] Remove redundant config options from blueprint YAML

[duncanmcclean]

This pull request aims to prevent config options from being saved in field configs when the values match the default config values for the fieldtype.

This PR also does a little bit of refactoring, moving all the Arr::get()/unset stuff into the ->reject() method. I've also added some tests to ensure my changes don't break anything. 😄

Closes #9431.

[jasonvarga]

Can you see if you can get icon: null and instructions: null get filtered out of replicator/bard groups/sets? Seems relevant to this PR.

          -
            handle: replicator_field
            field:
              type: replicator
              display: 'Replicator Field'
              sets:
                new_set_group:
                  display: 'New Set Group'
                  instructions: null
                  icon: null
                  sets:
                    new_set:
                      display: 'New Set'
                      instructions: null
                      icon: null
                      fields: {  }