Revert "[CodeCompletion] Add the initial support for code completing import declaration."

This reverts r30957 because it broke the following tests on Jenkins:

    SourceKit :: CodeComplete/complete_open.swift
    SourceKit :: CodeComplete/complete_test.swift

<rdar://problem/22120345> swift-incremental-RA #8289 failed to build

Swift SVN r30961

lib/IDE/CodeCompletion.cpp

@@ -667,7 +667,6 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
 
   enum class CompletionKind {
     None,
-    Import,
     DotExpr,
     PostfixExprBeginning,
     PostfixExpr,
@@ -834,7 +833,6 @@ public:
       SmallVectorImpl<StringRef> &Keywords) override;
 
   void completePoundAvailablePlatform() override;
-  void completeImportDecl() override;
 
   void addKeywords(CodeCompletionResultSink &Sink);
 
@@ -1020,27 +1018,6 @@ public:
     ExpressionSpecificDecls.insert(D);
   }
 
-  void addImportModuleNames() {
-    // FIXME: Add user-defined swift modules
-    SmallVector<clang::Module*, 20> Modules;
-    Ctx.getVisibleTopLevelClangeModules(Modules);
-    std::sort(Modules.begin(), Modules.end(),
-              [](clang::Module* LHS , clang::Module* RHS) {
-                return LHS->getTopLevelModuleName().compare_lower(
-                  RHS->getTopLevelModuleName()) < 0;
-              });
-    for (auto *M : Modules) {
-      if (M->isAvailable() && !M->getTopLevelModuleName().startswith("_")) {
-        CodeCompletionResultBuilder Builder(Sink,
-                                            CodeCompletionResult::ResultKind::
-                                              Keyword,
-                                            SemanticContextKind::None);
-        Builder.addTextChunk(M->getTopLevelModuleName());
-        Builder.addTypeAnnotation("Module");
-      }
-    }
-  }
-
   SemanticContextKind getSemanticContext(const Decl *D,
                                          DeclVisibilityKind Reason) {
     if (ForcedSemanticContext)
@@ -2479,11 +2456,6 @@ void CodeCompletionCallbacksImpl::completeCaseStmtDotPrefix() {
   CurDeclContext = P.CurDeclContext;
 }
 
-void CodeCompletionCallbacksImpl::completeImportDecl() {
-  Kind = CompletionKind::Import;
-  CurDeclContext = P.CurDeclContext;
-}
-
 void CodeCompletionCallbacksImpl::completeNominalMemberBeginning(
     SmallVectorImpl<StringRef> &Keywords) {
   assert(!InEnumElementRawValue);
@@ -2572,7 +2544,6 @@ void CodeCompletionCallbacksImpl::addKeywords(CodeCompletionResultSink &Sink) {
   case CompletionKind::AttributeDeclParen:
   case CompletionKind::AttributeBegin:
   case CompletionKind::PoundAvailablePlatform:
-  case CompletionKind::Import:
     break;
 
   case CompletionKind::PostfixExprBeginning:
@@ -2790,10 +2761,6 @@ void CodeCompletionCallbacksImpl::doneParsing() {
     Lookup.getPoundAvailablePlatformCompletions();
     break;
   }
-  case CompletionKind::Import: {
-    Lookup.addImportModuleNames();
-    break;
-  }
   }
 
   if (Lookup.RequestedCachedResults) {