Revert "PERF: Use Oj for serializing JSON. (#7780)"

This commit broke discourse-prometheus.

This reverts commit b4df8c5466.

config/initializers/100-oj.rb

@@ -1,6 +1,4 @@
 # frozen_string_literal: true
 
-Oj.optimize_rails
-
 # Not sure why it's not using this by default!
 MultiJson.engine = :oj

spec/integrity/oj_spec.rb

@@ -1,19 +0,0 @@
-# frozen_string_literal: true
-
-require "rails_helper"
-
-describe 'Oj' do
-  it "is enabled" do
-    classes = Set.new
-    tracer = TracePoint.new(:c_call) { |tp| classes << tp.defined_class }
-    tracer.enable { ActiveModel::ArraySerializer.new([1, 2, 3]).to_json }
-
-    expect(classes).to include(Oj::Rails::Encoder)
-  end
-
-  it "escapes HTML entities the same as ActiveSupport" do
-    expect("<b>hello</b>".to_json).to eq("\"\\u003cb\\u003ehello\\u003c/b\\u003e\"")
-    expect('"hello world"'.to_json). to eq('"\"hello world\""')
-    expect("\u2028\u2029><&".to_json).to eq('"\u2028\u2029\u003e\u003c\u0026"')
-  end
-end