Delete .lgtm.yml

LGTM has been acquired by Github, and all its features is replaced by Github Code Scanning.
So seems .lgtm.yml is no longer used.

(cherry picked from commit ee6cec1649b66f153d070a8171273370c4ef91a5)

.lgtm.yml

@@ -1,7 +0,0 @@
-extraction:
-  cpp:
-    after_prepare:
-      - pip3 install scons
-      - PATH="/opt/work/.local/bin:$PATH"
-    index:
-      build_command: scons -j2