[bolt] Don't call llvm::raw_string_ostream::flush() (NFC)

Don't call raw_string_ostream::flush(), which is essentially a no-op.
As specified in the docs, raw_string_ostream is always unbuffered.
( 65b13610a5 for further reference )

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -4245,7 +4245,6 @@ void RewriteInstance::addBoltInfoSection() {
          << "command line:";
   for (int I = 0; I < Argc; ++I)
     DescOS << " " << Argv[I];
-  DescOS.flush();
 
   // Encode as GNU GOLD VERSION so it is easily printable by 'readelf -n'
   const std::string BoltInfo =
@@ -4268,7 +4267,6 @@ void RewriteInstance::encodeBATSection() {
   raw_string_ostream DescOS(DescStr);
 
   BAT->write(*BC, DescOS);
-  DescOS.flush();
 
   const std::string BoltInfo =
       BinarySection::encodeELFNote("BOLT", DescStr, BinarySection::NT_BOLT_BAT);