[Instrumentation] Remove an unused local variable (NFC) (llvm#138383)

Author: kazutakahirata

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

@@ -1938,8 +1938,6 @@ void InstrLowerer::emitVNodes() {
 }
 
 void InstrLowerer::emitNameData() {
-  std::string UncompressedData;
-
   if (ReferencedNames.empty())
     return;