ci: remove LGTM

As LGTM will be closed at the end of the year 2022 we are going to remove it
from the CI and instead use CodeQL which is essentially the successor of it fully integrated into Github actions.

See the related blog post https://github.blog/2022-08-15-the-next-step-for-lgtm-com-github-code-scanning/

.lgtm.yml

@@ -1,23 +0,0 @@
-extraction:
-  cpp:
-    prepare:
-       packages:
-         - build-essential
-         - cmake
-         - libfftw3-dev
-         - libmbedtls-dev
-         - libpcsclite-dev
-         - libboost-program-options-dev
-         - libconfig++-dev
-         - libsctp-dev
-         - libuhd-dev
-         - libzmq3-dev
-    configure:
-      command:
-        - mkdir build
-        - cd build
-        - cmake ..
-    index:
-      build_command:
-        - cd build
-        - make