melpa-packages: remove llvm-mode override

This package was removed from melpa, so there's no value in overriding
it, and in fact it won't evaluate. :)

pkgs/applications/editors/emacs-modes/melpa-packages.nix

@@ -131,9 +131,6 @@ self:
       # upstream issue: mismatched filename
       link-hint = markBroken super.link-hint;
 
-      # part of a larger package
-      llvm-mode = dontConfigure super.llvm-mode;
-
       # upstream issue: missing file header
       maxframe = markBroken super.maxframe;