[nfc]remove unused variable after pr/81691 (llvm#82578)

* `N` became unused after [pull request 81691](llvm#81691)
* This should fix the build bot failure of `unused variable`
https://lab.llvm.org/buildbot/#/builders/77/builds/34840

llvm/lib/ProfileData/InstrProfWriter.cpp

@@ -456,7 +456,6 @@ Error InstrProfWriter::writeImpl(ProfOStream &OS) {
   Header.BinaryIdOffset = 0;
   Header.TemporalProfTracesOffset = 0;
   Header.VTableNamesOffset = 0;
-  int N = sizeof(IndexedInstrProf::Header) / sizeof(uint64_t);
 
   // Only write out the first four fields. We need to remember the offset of the
   // remaining fields to allow back patching later.