Do not re-register plugin pragmas.

The clang::Preprocessor::RegisterBuiltinPragmas already registered them

This patch avoids an assertion when re-adding the same pragma twice.

lib/Interpreter/IncrementalParser.cpp

@@ -218,14 +218,6 @@ static void HandlePlugins(CompilerInstance& CI,
         Consumers.push_back(std::move(PluginConsumer));
     }
   }
-
-  // Copied from Lex/Pragma.cpp
-
-  // Pragmas added by plugins
-  for (PragmaHandlerRegistry::iterator it = PragmaHandlerRegistry::begin(),
-          ie = PragmaHandlerRegistry::end(); it != ie; ++it) {
-     CI.getPreprocessor().AddPragmaHandler(it->instantiate().release());
-  }
 }
 
 namespace cling {