[BOLT][NFC] Fix unused variable warning (#93323)

This fixes

```
warning: variable 'AbbrDecl' set but not used [-Wunused-but-set-variable]

   76 |     if (const DWARFAbbreviationDeclaration *AbbrDecl =
```

Author: sayhaan

bolt/lib/Rewrite/DWARFRewriter.cpp

@@ -73,8 +73,7 @@ static void printDie(DWARFUnit &DU, uint64_t DIEOffset) {
   DWARFDataExtractor DebugInfoData = DU.getDebugInfoExtractor();
   DWARFDebugInfoEntry DIEEntry;
   if (DIEEntry.extractFast(DU, &DIEOffset, DebugInfoData, NextCUOffset, 0)) {
-    if (const DWARFAbbreviationDeclaration *AbbrDecl =
-            DIEEntry.getAbbreviationDeclarationPtr()) {
+    if (DIEEntry.getAbbreviationDeclarationPtr()) {
       DWARFDie DDie(&DU, &DIEEntry);
       printDie(DDie);
     } else {