Removes es6.

[ghost]

Refs inveniosoftware/invenio-records-resources#305

[ghost]

The failing tests do not seem to be connected to my changes:

Run python -m pip install setuptools">=57.0.0,<58.0.0"
Collecting setuptools<58.0.0,>=57.0.0
  Downloading setuptools-57.5.0-py3-none-any.whl (819 kB)
ERROR: launchpadlib 1.10.13 requires testresources, which is not installed.
Installing collected packages: setuptools
Successfully installed setuptools-57.5.0
Collecting pip
  Downloading pip-22.1-py3-none-any.whl (2.1 MB)
Collecting py
  Downloading py-1.11.0-py2.py3-none-any.whl (98 kB)
Collecting wheel
  Downloading wheel-0.37.1-py2.py3-none-any.whl (35 kB)
Collecting requirements-builder
  Downloading requirements_builder-0.4.4-py2.py3-none-any.whl (9.0 kB)
Requirement already satisfied, skipping upgrade: click>=7.0 in /usr/lib/python3/dist-packages (from requirements-builder) (7.0)
Collecting mock<4,>=1.3.0
  Downloading mock-3.0.5-py2.py3-none-any.whl (25 kB)
Requirement already satisfied, skipping upgrade: six in /usr/lib/python3/dist-packages (from mock<4,>=1.3.0->requirements-builder) (1.14.0)
Installing collected packages: pip, py, wheel, mock, requirements-builder
Successfully installed mock-3.0.5 pip-22.1 py-1.11.0 requirements-builder-0.4.4 wheel-0.37.1
Usage: requirements-builder [OPTIONS] [SETUP]
Try "requirements-builder --help" for help.

Error: Invalid value for "--level" / "-l": invalid choice: . (choose from min, pypi, dev)
Error: Process completed with exit code 2.

[ntarocco]

Thanks! Done here: 8930862