Frontend: Remove support for -experimental-spi-imports flag.

Its functionality has been superseded by `@_spiOnly import`. There are no
longer any known clients and the flag was already unsupported in Swift 6, so
the functionality is now removed (but the flag is only deprecated for Swift 5).

Resolves rdar://136867210.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -279,27 +279,6 @@ static void printImports(raw_ostream &out,
     return importSet;
   };
 
-  // With -experimental-spi-imports:
-  // When printing the private or package swiftinterface file, print implementation-only
-  // imports only if they are also SPI. First, list all implementation-only imports and
-  // filter them later.
-  llvm::SmallSet<ImportedModule, 4, ImportedModule::Order> ioiImportSet;
-  if (!Opts.printPublicInterface() && Opts.ExperimentalSPIImports) {
-
-    SmallVector<ImportedModule, 4> ioiImports, allImports;
-    M->getImportedModules(ioiImports,
-                          ModuleDecl::ImportFilterKind::ImplementationOnly);
-
-    // Only consider modules imported consistently as implementation-only.
-    ImportSet allImportSet = getImports(allImportFilter);
-
-    for (auto import: ioiImports)
-      if (allImportSet.count(import) == 0)
-        ioiImportSet.insert(import);
-
-    allImportFilter |= ModuleDecl::ImportFilterKind::ImplementationOnly;
-  }
-
   /// Collect @_spiOnly imports that are not imported elsewhere publicly.
   ImportSet spiOnlyImportSet;
   if (!Opts.printPublicInterface()) {
@@ -367,13 +346,6 @@ static void printImports(raw_ostream &out,
     llvm::SmallSetVector<Identifier, 4> spis;
     M->lookupImportedSPIGroups(importedModule, spis);
 
-    // Only print implementation-only imports which have an SPI import.
-    if (ioiImportSet.count(import)) {
-      if (spis.empty())
-        continue;
-      out << "@_implementationOnly ";
-    }
-
     if (exportedImportSet.count(import))
       out << "@_exported ";