snippetpixie: remove elementary-gtk-theme from buildInputs

Adding the theme to buildInputs does not ensure it to present
in XDG_DATA_DIRS. The app does not hardcode GTK theme so if
user has some GTK theme installed this won't work anyway.

pkgs/tools/text/snippetpixie/default.nix

···
     json-glib
     xorg.libXtst
     pantheon.granite
-    pantheon.elementary-gtk-theme
   ];
 
   doCheck = true;