chore(set_theory/game/domineering): extract repeated goal into lemma and golf (#11298)
chore(set_theory/game/domineering): extract repeated goal into lemma and golf (#11298)

`fst_pred_mem_erase_of_mem_right` and `snd_pred_mem_erase_of_mem_left` were common subgoals that appeared in two lemmas each.






leanprover-community/mathlib,leanprover-community/mathlib