lgtm: use the system version of meson

LGTM was upgraded to Ubuntu Eoan, so the meson there is pretty new.
Also, the pip command didn't work anyway, since it didn't attempt to
update the previously installed meson.

.lgtm.yml

@@ -5,16 +5,10 @@ extraction:
   cpp:
     prepare:
       packages:
-        - python3-pip
-        - python3-setuptools
-        - python3-wheel
         - libpwquality-dev
         - libfdisk-dev
         - libp11-kit-dev
         - libssl-dev
-    after_prepare:
-      - pip3 install meson
-      - export PATH="$HOME/.local/bin/:$PATH"
     before_index:
       # /tmp has `noexec` set in the LGTM infra which breaks meson's
       # compilation checks. Let's temporarily override TMPDIR to /var/tmp which