[ctx_prof] Remove Buffer field in PGOCtxProfWriter

Not needed anymore after PR llvm#92983, and moreover, creating the same issue c49bc1a fixed in `BitcodeWriter.cpp`.

Author: mtrofin

llvm/include/llvm/ProfileData/PGOCtxProfWriter.h

@@ -60,7 +60,6 @@ enum PGOCtxProfileBlockIDs {
 /// example, value profiling would produce a new record with a new record ID,
 /// containing the profiled values (much like the counters)
 class PGOCtxProfileWriter final {
-  SmallVector<char, 1 << 20> Buff;
   BitstreamWriter Writer;
 
   void writeCounters(const ctx_profile::ContextNode &Node);