Add a parameter to the forget_skill method to make a Pokémon completely forget a move

MR Description

• This MR adds an optional parameter to the forget_skill method. This parameter allows the user to also delete a move from the current @skill_learnt attribute of a Pokémon. This is useful for Pokémon like Rotom which completely forget the moves it has learned in one of its motor form when it changes form and go back to the base form.

Tests to realize

• Use the forget_skill method on a Pokémon with the delete_from_learnt: to true then check the skill_learnt attribute of said Pokémon afterwards: the move should have disappeared from the Array