Simplify calculate_checksum! method

The `update_verification_state!` method also sets
`verification_checksum`, so we don't need to do it here.

ee/lib/gitlab/geo/replicable_model.rb

@@ -46,13 +46,13 @@ module Gitlab
         raise NotImplementedError, 'There is no Replicator defined for this model'
       end
 
-      # Clear model verification checksum and force recalculation
+      # Returns a checksum of the file (assumed to be a "blob" type)
+      #
+      # @return [String] SHA256 hash of the carrierwave file
       def calculate_checksum!
-        self.verification_checksum = nil
-
-        return unless needs_checksum?
+        return unless checksummable?
 
-        self.verification_checksum = self.class.hexdigest(replicator.carrierwave_uploader.path)
+        self.class.hexdigest(replicator.carrierwave_uploader.path)
       end
 
       # Checks whether model needs checksum to be performed

ee/spec/models/packages/package_file_spec.rb

@@ -7,21 +7,16 @@ RSpec.describe Packages::PackageFile, type: :model do
   describe '#calculate_checksum!' do
     let(:package_file) { create(:conan_package_file, :conan_recipe_file) }
 
-    it 'sets `verification_checksum` to SHA256 sum of the file' do
+    it 'returns SHA256 sum of the file' do
       expected = Digest::SHA256.file(package_file.file.path).hexdigest
 
-      expect { package_file.calculate_checksum! }
-        .to change { package_file.verification_checksum }.from(nil).to(expected)
+      expect(package_file.calculate_checksum!).to eq(expected)
     end
 
-    it 'sets `checksum` to nil for a non-existent file' do
-      checksum = Digest::SHA256.file(package_file.file.path).hexdigest
-      package_file.verification_checksum = checksum
-
+    it 'returns nil for a non-existent file' do
       allow(package_file).to receive(:file_exist?).and_return(false)
 
-      expect { package_file.calculate_checksum! }
-        .to change { package_file.verification_checksum }.from(checksum).to(nil)
+      expect(package_file.calculate_checksum!).to eq(nil)
     end
   end