Remove DesignAtVersion.instantiate method

Removing this method as it is unused.

app/models/design_management/design_at_version.rb

@@ -21,10 +21,6 @@ module DesignManagement
       @design, @version = design, version
     end
 
-    def self.instantiate(attrs)
-      new(**attrs).tap { |obj| obj.validate! }
-    end
-
     # The ID, needed by GraphQL types and as part of the Lazy-fetch
     # protocol, includes information about both the design and the version.
     #

spec/models/design_management/design_at_version_spec.rb

@@ -274,29 +274,6 @@ RSpec.describe DesignManagement::DesignAtVersion do
     build(:design_at_version, design: design, version: version).id
   end
 
-  describe '.instantiate' do
-    context 'when attrs are valid' do
-      subject do
-        described_class.instantiate(design: design, version: version)
-      end
-
-      it { is_expected.to be_a(described_class).and(be_valid) }
-    end
-
-    context 'when attrs are invalid' do
-      subject do
-        described_class.instantiate(
-          design: create(:design),
-          version: create(:design_version)
-        )
-      end
-
-      it 'raises a validation error' do
-        expect { subject }.to raise_error(ActiveModel::ValidationError)
-      end
-    end
-  end
-
   describe '.lazy_find' do
     let!(:version_a) do
       create(:design_version, designs: create_list(:design, 3, issue: issue))