Fixed bug: wrong attribute is removed in label constructor

[bsekachev]

Motivation and context

Resolved #2411

How has this been tested?

Manual testing
@dvkruchinin Please, write a test for the issue
Scenario is:

1. Open the create task page
2. Go to Raw labels editor
3. Insert there:

[
  {
    "name": "person",
    "color": "#ff6037",
    "attributes": [
      {
        "name": "lower_body",
        "input_type": "select",
        "mutable": true,
        "values": [
          "__undefined__",
          "long",
          "short",
          "n/a"
        ]
      },
      {
        "name": "hair_color",
        "input_type": "select",
        "mutable": true,
        "values": [
          "__undefined__",
          "black",
          "brown",
          "blond",
          "grey",
          "other",
          "n/a"
        ]
      },
      {
        "name": "cellphone",
        "input_type": "select",
        "mutable": true,
        "values": [
          "__undefined__",
          "yes",
          "no",
          "n/a"
        ]
      }
    ]
  }
]

4. Press done
5. Go to constructor tab, be sure that the label "person" appeared there
6. Start editing it
7. Try to remove the average attribute "hair_color", be sure it has been deleted
8. Try to remove the latest attribute "lower_body", be sure it has been deleted

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[dvkruchinin]

Hi. Yes, of course. I will write a test to test this fix.

[coveralls]

Coverage remained the same at 62.831% when pulling 546e30b on bs/fixed_issue_2411 into a9b2817 on develop.