Remove LGTM config

LGTM.com is no longer available since it was superseded by CodeQL.

.lgtm.yml

@@ -1,13 +0,0 @@
-# SPDX-FileCopyrightText: 2020 yuzu Emulator Project
-# SPDX-License-Identifier: GPL-2.0-or-later
-
-path_classifiers:
-  library: "externals"
-extraction:
-  cpp:
-    prepare:
-      packages:
-      - "libsdl2-dev"
-      - "qtmultimedia5-dev"
-      - "libtbb-dev"
-      - "libjack-jackd2-dev"