ASTPrinter: Remove printWithCompatibilityFeatureChecks() from printSynthesizedExtensionIfNeeded()

This path was not exercised in the test suite, and when the
PrimaryAssociatedTypes feature finally (incorrectly) triggered it,
it would print '#endifextension'.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -650,17 +650,8 @@ public:
         printer << " {}";
       };
 
-      bool printedNewline = false;
-      if (printOptions.PrintCompatibilityFeatureChecks) {
-        printedNewline =
-          printWithCompatibilityFeatureChecks(printer, curPrintOptions,
-                                              proto, printBody);
-      } else {
-        printBody();
-        printedNewline = false;
-      }
-      if (!printedNewline)
-        printer << "\n";
+      printBody();
+      printer << "\n";
     }
   }