l_mainmenu.h: remove unused l_get_dirlist function

The commit
8f9af573 "Add core.get_dir_list" by @ShadowNinja
has removed the implementation of the l_get_dirlist function and all its usages
from the l_mainmenu.cpp file, but hasn't removed it from the header file.

The reason why this hasn't been detected earlier is that C++ has this interesting
feature to still make it possible to create instances of classes whose never used
private methods are declared but not defined.

src/script/lua_api/l_mainmenu.h

@@ -119,8 +119,6 @@ class ModApiMainMenu : public ModApiBase {
 
 	static int l_get_texturepath_share(lua_State *L);
 
-	static int l_get_dirlist(lua_State *L);
-
 	static int l_create_dir(lua_State *L);
 
 	static int l_delete_dir(lua_State *L);