What would someone saying ... ンなこと mean?

Question

I assume it's some alternative for 'nevermind'?

・・・ンなこと
いーからさっさと
帰ろーぜっ

Answer 1

んなこと is just an abbreviated form of そんなこと

Answer 2

…ンなこと is the same as そんなこと. そんなこと means "about that".
In English, 気にする is "worry". So the translation is:

Don't worry about that.