Frontend: Remove the -skip-import-in-public-interface flag.

This flag is unsafe since the compiler does not verify that the resulting
public interface will compile with the module import removed. The modern
alternative to this flag is `@_spiOnly import`. Since the flag is no longer
used by any projects it should be removed.

Resolves rdar://134351088.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -339,11 +339,6 @@ static void printImports(raw_ostream &out,
       continue;
     }
 
-    if (llvm::count(Opts.ModulesToSkipInPublicInterface,
-                    importedModule->getName().str())) {
-      continue;
-    }
-
     llvm::SmallSetVector<Identifier, 4> spis;
     M->lookupImportedSPIGroups(importedModule, spis);