Remove old UI

[nmanovic]

Motivation and context

Remove the deprecated UI source code. We are not going to support both versions.

How has this been tested?

Manual testing only.

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

[coveralls]

Pull Request Test Coverage Report for Build 6731

• 11 of 16 (68.75%) changed or added relevant lines in 3 files are covered.
• 105 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.05%) to 69.093%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/engine/urls.py	7	12	58.33%

Files with Coverage Reduction	New Missed Lines	%
src/frames.js	3	23.31%
src/session.js	102	54.35%

Totals
Change from base Build 6672:	0.05%
Covered Lines:	11145
Relevant Lines:	15620

---

💛 - Coveralls

[bsekachev]

Karma and qunit probably aren't needed anymore in Dockerfile.ci

[nmanovic]

@azhavoro , could you please look and test?