I'm not sure if you mean you don't need the year in the result or if you also don't want to group by the year. So here's one version with the year and one without:
var result1 = Accounts.GroupBy(a => new { Year = a.CreatedDate.Year, Month = a.CreatedDate.Month })
.Select(g => new { Year = g.Key.Year, Month = g.Key.Month, Count = g.Count() });
var result2 = Accounts.GroupBy(a => a.CreatedDate.Month)
.Select(g => new { Month = g.Key, Count = g.Count() });