remove redundant flake8 check

it's already in pre-commit-hooks

flake.nix

@@ -70,11 +70,6 @@
             };
           };
         };
-        format = pkgs.runCommand "check-format" {} ''
-          ${pkgs.python3Packages.flake8}/bin/flake8 \
-              --exclude ${./.}/app/signup/migrations/ ${./.}/app
-          mkdir -p $out
-        '';
         build-vm = self.nixosConfigurations.vm.config.system.build.vm;
       };