ui: Remove unused zoom slider portion of the minimap panel

This allows us to claim back 10px for the rest of the sidebar.

data/themes/_initial.cfg

@@ -280,7 +280,7 @@
         id=minimap-panel
         #ref=terrain-description-box-botleft
         image=themes/classic/minimap.png
-        rect="=,=+6,+181,+265"
+        rect="=,=+6,+181,+255"
         xanchor=right
         yanchor=fixed
     [/panel]
@@ -298,7 +298,7 @@
         image=button_square/button_square_25
         items=zoomdefault
         auto_tooltip=yes
-        rect="=+1,+13,+25,+25"
+        rect="=+1,+3,+25,+25"
         xanchor=right
         yanchor=fixed
     [/action]