Is the use of 'of' necessary in 'all of those who'?
Solution 1:
In the case of "all of those" vs. "all those", it's a matter of personal preference. Personally, I would use "all those"; I think it flows better.
This Google Ngram also shows preference for "all those":
I included both "that" and "who", but as others have said, "who" should be used (i.e. it's not strictly incorrect to use "that") when referring to people or groups thereof.