Use hash lookup for deleting existing associations from `target`

`Array#delete` searches for all occurrences in the `target` array. When performing `dependent: :destroy` in active_record/associations/collection_association, the loop becomes O(N^2). It is particularly slow when destroying large amount of associations (e.g. 10K records).

Either `Hash` or `Set` can optimize the loop to O(N). `Hash` is slightly faster in this simple usage.

```ruby
class Dummy; end

num = 10_000
test1a = num.times.map { Dummy.new }; nil
test1b = test1a.dup
test2a = num.times.map { Dummy.new }; nil
test2b = test2a.dup

Benchmark.ips do |x|
  x.config(stats: :bootstrap, confidence: 95)

  x.report("hash") do
    hash = test1a.group_by { |r| r }
    test1b.select! { |r| !hash[r] }
  end
  x.report("array")  do
    test2a.each { |r| test2b.delete(r) }
  end
  x.compare!
end
```

```
Calculating -------------------------------------
                hash    11.000  i/100ms
               array     1.000  i/100ms
-------------------------------------------------
                hash    114.586  (±16.6%) i/s -    561.000
               array      1.710k (±10.3%) i/s -      8.377k

Comparison:
               array:     1710.4 i/s
                hash:      114.6 i/s - 14.93x slower
```

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