luacheck: remove unneeded comment

serpent module has been dropped in commit
b53cb2ae
"console: drop unused serpent module", but comment that belong to module
was left in luacheck config.

.luacheckrc

@@ -30,7 +30,6 @@ include_files = {
 
 exclude_files = {
     "build/**/*.lua",
-    -- Third-party source code.
     "test-run/**/*.lua",
     "test/app/*.test.lua",
     "test/box/**/*.lua",