Use clamdscan by default.

This eliminates some unnecessary config from govuk-helm-charts and makes
asset-manager more robust against the clam binaries moving to a slightly
different directory in PATH (which was the cause of a recent production
outage).

Remove a unhelpful change-detector test for the default value.

lib/govuk_configuration.rb

@@ -23,6 +23,6 @@ def draft_assets_host
   end
 
   def clamscan_path
-    @env.fetch("ASSET_MANAGER_CLAMSCAN_PATH", "govuk_clamscan")
+    @env.fetch("ASSET_MANAGER_CLAMSCAN_PATH", "clamdscan")
   end
 end

spec/lib/govuk_configuration_spec.rb

@@ -63,14 +63,6 @@
         expect(config.clamscan_path).to eq("alternative-path")
       end
     end
-
-    context "when environment does not include an ASSET_MANAGER_CLAMSCAN_PATH value" do
-      let(:env) { {} }
-
-      it "returns govuk_clamscan" do
-        expect(config.clamscan_path).to eq("govuk_clamscan")
-      end
-    end
   end
 
   describe "#draft_assets_host" do