[clang-scan-deps] NFC, remove outdated implementation comment

There's no need to purge symlinked entries in the FileManager,
as the new FileEntryRef API allows us to compute dependencies more
accurately when the FileManager is reused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370493 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -87,7 +87,6 @@ public:
 
     // Use the dependency scanning optimized file system if we can.
     if (DepFS) {
-      // FIXME: Purge the symlink entries from the stat cache in the FM.
       const CompilerInvocation &CI = Compiler.getInvocation();
       // Add any filenames that were explicity passed in the build settings and
       // that might be opened, as we want to ensure we don't run source