Get rid of unused Sqlite3Adapter#use_insert_returning?

It was introduced in rails#49290
likely to do the same as PostgresqlAdapter, but it's not used
anywhere and there not really any reason to have that option.

Postgres has it because INSERT RETURNING can't be used in some
corner cases, see: rails#5698

Author: byroot

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -127,7 +127,6 @@ def initialize(...)
           results_as_hash: true,
           default_transaction_mode: :immediate,
         )
-        @use_insert_returning = @config.key?(:insert_returning) ? self.class.type_cast_config_to_boolean(@config[:insert_returning]) : true
       end
 
       def database_exists?
@@ -434,10 +433,6 @@ def shared_cache? # :nodoc:
         @config.fetch(:flags, 0).anybits?(::SQLite3::Constants::Open::SHAREDCACHE)
       end
 
-      def use_insert_returning?
-        @use_insert_returning
-      end
-
       def get_database_version # :nodoc:
         SQLite3Adapter::Version.new(query_value("SELECT sqlite_version(*)", "SCHEMA"))
       end