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
```

1 files changed, 2 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index ed2e6d1ae4..65ef9f617a 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -392,7 +392,8 @@ module ActiveRecord
           records.each { |record| callback(:before_remove, record) }
 
           delete_records(existing_records, method) if existing_records.any?
-          records.each { |record| target.delete(record) }
+          hashed_records = records.group_by { |record| record }
+          target.select! { |record| !hashed_records[record] }
 
           records.each { |record| callback(:after_remove, record) }
         end