vimPlugins.notmuch-vim: move to non-generated

pkgs/applications/editors/vim/plugins/non-generated/notmuch-vim/default.nix

···
+{ notmuch }: notmuch.vim

pkgs/applications/editors/vim/plugins/overrides.nix

···
   neovim-unwrapped,
   nim1,
   nodejs,
-  notmuch,
   openscad,
   openssh,
   parinfer-rust,
···
 
   NotebookNavigator-nvim = super.NotebookNavigator-nvim.overrideAttrs {
   };
-
-  notmuch-vim = notmuch.vim;
 
   nterm-nvim = super.nterm-nvim.overrideAttrs {
     dependencies = [ self.aniseed ];