mailcore2: remove git conflict line, still doesn't build

pkgs/development/libraries/mailcore2/default.nix

···
        --replace "/usr/include/libxml2" "${libxml2.dev}/include/libxml2"
     substituteInPlace src/core/basetypes/MCHTMLCleaner.cpp \
       --replace buffio.h tidybuffio.h
->>>>>>> master
   '';
 
   cmakeFlags = [