updating the screen in tap handler of readerfooter is not needed

Was a hack anyway, c.f.
https://github.com/koreader/koreader/pull/1306#discussion_r21057461

Seems to be not needed anymore. If some areas are now not refreshed
anymore, that ought to be fixed in another place.

This also gets rid of redundant screen flashes.

frontend/apps/reader/modules/readerfooter.lua

@@ -267,20 +267,13 @@ function ReaderFooter:onTapFooter(arg, ges)
             self.mode = 0
         end
         self:applyFooterMode()
+        G_reader_settings:saveSetting("reader_footer_mode", self.mode)
     end
     if self.pageno then
         self:updateFooterPage()
     else
         self:updateFooterPos()
     end
-    local region = Geom:new{
-        x = 0,
-        y = Screen:getHeight() - self.height,
-        w = Screen:getWidth(),
-        h = self.height
-    }
-    UIManager:setDirty(self.view.dialog, "partial", region)
-    G_reader_settings:saveSetting("reader_footer_mode", self.mode)
     return true
 end