remove unused functions 'mrb_strerrno' and 'mrb_bug_errno'.

'strerror' is not supported on the freestanding environment ('strerror' is declared in 'string.h'. And 'string.h' is not provided for freestanding environment).
And 'mrb_bug_errno' is not called by any other code.
So 'mrb_strerrno' is called just only by 'mrb_bug_errno'.