[BOLT] Fix a warning

This patch fixes:

  bolt/lib/Rewrite/DWARFRewriter.cpp:187:18: error: private field
  'Rewriter' is not used [-Werror,-Wunused-private-field]

Note that:

  commit 61df854
  Author: Sayhaan Siddiqui <49014204+sayhaan@users.noreply.github.com>
  Date:   Mon Jun 10 10:46:20 2024 -0700

removed the last use of Rewriter.

bolt/lib/Rewrite/DWARFRewriter.cpp

@@ -184,7 +184,6 @@ namespace bolt {
 /// Emits debug information into .debug_info or .debug_types section.
 class DIEStreamer : public DwarfStreamer {
   DIEBuilder *DIEBldr;
-  DWARFRewriter &Rewriter;
   GDBIndex &GDBIndexSection;
 
 private:
@@ -285,7 +284,7 @@ class DIEStreamer : public DwarfStreamer {
               raw_pwrite_stream &OutFile,
               DWARFLinkerBase::MessageHandlerTy Warning)
       : DwarfStreamer(OutFileType, OutFile, Warning), DIEBldr(DIEBldr),
-        Rewriter(Rewriter), GDBIndexSection(GDBIndexSection) {};
+        GDBIndexSection(GDBIndexSection) {};
 
   using DwarfStreamer::emitCompileUnitHeader;