Remove calls to raw_svector_ostream::flush()

This function is deleted since LLVM r244928.

Swift SVN r31798

lib/IDE/CodeCompletionCache.cpp

@@ -377,7 +377,6 @@ static std::string getName(StringRef cacheDirectory,
   llvm::APInt(64, uint64_t(hash)).toStringUnsigned(hashStr, /*Radix*/ 36);
   OSS << "-" << hashStr << ".completions";
 
-  OSS.flush();
   return name.str();
 }