Remove inherited themes from index.theme

The FreeDesktop.org Icon Theme Spec has been
updated stating that inherited themes should
be present on the system (considered dependencies).

The currently listed themes in elementary-xfce's
`Inherits` line are either no longer developed and
missing modern icons (gnome), or have reduced the scope
of their theme to focus on their OS (Adwaita, elementary).

elementary-xfce should currently be robust enough that
it should already include the icons found in these themes.
So it no longer seems very necessary to keep these, and
removing them will prevent unnecessary themes from being
forced as dependencies for users who may not want them.

Fixes shimmerproject#458

elementary-xfce/index.theme

@@ -1,7 +1,7 @@
 [Icon Theme]
 Name=elementary Xfce
 Comment=Smooth modern theme for Xfce
-Inherits=elementary,Adwaita,gnome,hicolor
+Inherits=hicolor
 
 Example=directory-x-normal