Remove default hotkey for toggle zone display

It is too easy to accidentally hide your zones and the 'A' hotkey is
intuitively mapped to Add Footprint, to match Add Symbol in eeschema.

This won't change current hotkey customizations but will help prevent
future user error.  Users can still map the action to a hotkey if the
new toggle is useful to their workflow

(cherry picked from commit 4d899d7820)

pcbnew/tools/pcb_actions.cpp

@@ -658,7 +658,7 @@ TOOL_ACTION PCB_ACTIONS::placeTarget( "pcbnew.EditorControl.placeTarget",
 
 TOOL_ACTION PCB_ACTIONS::placeFootprint( "pcbnew.EditorControl.placeFootprint",
         AS_GLOBAL,
-        'O', LEGACY_HK_NAME( "Add Footprint" ),
+        'A', LEGACY_HK_NAME( "Add Footprint" ),
         _( "Add Footprint" ), _( "Add a footprint" ),
         BITMAPS::module, AF_ACTIVATE );
 
@@ -875,7 +875,7 @@ TOOL_ACTION PCB_ACTIONS::zoneDisplayTriangulated( "pcbnew.Control.zoneDisplayTes
 
 TOOL_ACTION PCB_ACTIONS::zoneDisplayToggle( "pcbnew.Control.zoneDisplayToggle",
         AS_GLOBAL,
-        'A', "",
+        0, "",
         _( "Toggle Zone Display" ),
         _( "Cycle between showing zone fills and just their outlines" ),
         BITMAPS::show_zone );