Remove LGTM.com configuration file

[DimitriPapadopoulos]

LGTM.com is being deprecated and replaced by GitHub code analysis: The next step for LGTM.com: GitHub code scanning!

See also:

• #909
• #1127

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [X] GitHub Actions have all passed
• [X] Test coverage is 100% (Codecov passes)