freecad: remove duplicated cmakeFlag

(cherry picked from commit c002f9a)

Author: qbisi

pkgs/by-name/fr/freecad/package.nix

@@ -150,7 +150,6 @@ freecad-utils.makeCustomizable (
 
     cmakeFlags = [
       "-Wno-dev" # turns off warnings which otherwise makes it hard to see what is going on
-      "-DBUILD_FLAT_MESH:BOOL=ON"
       "-DBUILD_DRAWING=ON"
       "-DBUILD_FLAT_MESH:BOOL=ON"
       "-DINSTALL_TO_SITEPACKAGES=OFF"