Remove love.dropbegan and love.dropcompleted callbacks. Closes #1908

They don't do what most people might assume they do (they happen at the same time as the user completes a drag-and-drop operation and they surround the dropfile events, instead of starting when the user initiates the operation). Also their functionality can be replicated without needing dedicated love event callbacks.

src/modules/event/sdl/Event.cpp

@@ -414,12 +414,6 @@ Message *Event::convert(const SDL_Event &e)
 		}
 		SDL_free(e.drop.file);
 		break;
-	case SDL_DROPBEGIN:
-		msg = new Message("dropbegan");
-		break;
-	case SDL_DROPCOMPLETE:
-		msg = new Message("dropcompleted");
-		break;
 	case SDL_QUIT:
 	case SDL_APP_TERMINATING:
 		msg = new Message("quit");

src/modules/love/callbacks.lua

@@ -115,12 +115,6 @@ function love.createhandlers()
 		directorydropped = function (dir)
 			if love.directorydropped then return love.directorydropped(dir) end
 		end,
-		dropbegan = function ()
-			if love.dropbegan then return love.dropbegan() end
-		end,
-		dropcompleted = function ()
-			if love.dropcompleted then return love.dropcompleted() end
-		end,
 		lowmemory = function ()
 			if love.lowmemory then love.lowmemory() end
 			collectgarbage()