ci: drop LGTM integration

LGTM is deprecated, remove it. We have CodeQL now.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.lgtm.yml

@@ -1,14 +0,0 @@
-# vi: set ts=2 sw=2:
-extraction:
-  cpp:
-    prepare:
-      packages:
-        - libelf-dev
-        - pkg-config
-    after_prepare:
-      # As the buildsystem detection by LGTM is performed _only_ during the
-      # 'configure' phase, we need to trick LGTM we use a supported build
-      # system (configure, meson, cmake, etc.). This way LGTM correctly detects
-      # that our sources are in the src/ subfolder.
-      - touch src/configure
-      - chmod +x src/configure