Remove redundant installation of libldap2-dev

[lravindr]

Removed the redundant installation of libldap2-dev library in the Dockerfile. This is fixing for #1852.

Motivation and context

How has this been tested?

Tested locally by pulling and checking if the build works.

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

[nmanovic]

Thanks for the PR!