[Driver] Remove unused getInstalledDir

The uses have been removed by commit 2b5cd8b

Author: MaskRay

clang/include/clang/Driver/Driver.h

@@ -424,11 +424,6 @@ class Driver {
     return ClangExecutable.c_str();
   }
 
-  /// Get the path to where the clang executable was installed.
-  const char *getInstalledDir() const {
-    return Dir.c_str();
-  }
-
   bool isSaveTempsEnabled() const { return SaveTemps != SaveTempsNone; }
   bool isSaveTempsObj() const { return SaveTemps == SaveTempsObj; }