Remove :oj_json feature flag

Removes the :oj_json feature flag and defaults to the
OJ gem for most JSON requirements.

config/feature_flags/development/oj_json.yml

----
-name: oj_json
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

lib/gitlab/json.rb

@@ -67,15 +67,6 @@ module Gitlab
         ::JSON.pretty_generate(object, opts)
       end
 
-      # Feature detection for using Oj instead of the `json` gem.
-      #
-      # @return [Boolean]
-      def enable_oj?
-        return false unless feature_table_exists?
-
-        Feature.enabled?(:oj_json, default_enabled: true)
-      end
-
       private
 
       # Convert JSON string into Ruby through toggleable adapters.
@@ -91,11 +82,7 @@ module Gitlab
       def adapter_load(string, *args, **opts)
         opts = standardize_opts(opts)
 
-        if enable_oj?
-          Oj.load(string, opts)
-        else
-          ::JSON.parse(string, opts)
-        end
+        Oj.load(string, opts)
       rescue Oj::ParseError, Encoding::UndefinedConversionError => ex
         raise parser_error.new(ex)
       end
@@ -120,11 +107,7 @@ module Gitlab
       #
       # @return [String]
       def adapter_dump(object, *args, **opts)
-        if enable_oj?
-          Oj.dump(object, opts)
-        else
-          ::JSON.dump(object, *args)
-        end
+        Oj.dump(object, opts)
       end
 
       # Generates JSON for an object but with fewer options, using toggleable adapters.
@@ -135,11 +118,7 @@ module Gitlab
       def adapter_generate(object, opts = {})
         opts = standardize_opts(opts)
 
-        if enable_oj?
-          Oj.generate(object, opts)
-        else
-          ::JSON.generate(object, opts)
-        end
+        Oj.generate(object, opts)
       end
 
       # Take a JSON standard options hash and standardize it to work across adapters
@@ -149,11 +128,8 @@ module Gitlab
       # @return [Hash]
       def standardize_opts(opts)
         opts ||= {}
-
-        if enable_oj?
-          opts[:mode] = :rails
-          opts[:symbol_keys] = opts[:symbolize_keys] || opts[:symbolize_names]
-        end
+        opts[:mode] = :rails
+        opts[:symbol_keys] = opts[:symbolize_keys] || opts[:symbolize_names]
 
         opts
       end
@@ -213,7 +189,7 @@ module Gitlab
       # @param object [Object]
       # @return [String]
       def self.call(object, env = nil)
-        if Gitlab::Json.enable_oj? && Feature.enabled?(:grape_gitlab_json, default_enabled: true)
+        if Feature.enabled?(:grape_gitlab_json, default_enabled: true)
           Gitlab::Json.dump(object)
         else
           Grape::Formatter::Json.call(object, env)