Merge bitcoin/bitcoin#26258: refactor: Remove unused CDataStream::rdbuf method

fabbbe3 Remove unused CDataStream::rdbuf method (MacroFake)

Pull request description:

  It is unused and seems unlikely to be ever used.

ACKs for top commit:
  theStack:
    Code-review ACK fabbbe3
  aureleoules:
    ACK fabbbe3

Tree-SHA512: 5804642658f96a0fb51482ebf3a062bb0f997c1e0527455afa4aceeeb6c1ad139a98b14a7c8a0909daba733a83bdc24fcadad45060ead4be6eb3dc3e66c129e2

Author: fanquake

src/streams.h

@@ -269,7 +269,6 @@ class CDataStream
     // Stream subset
     //
     bool eof() const             { return size() == 0; }
-    CDataStream* rdbuf()         { return this; }
     int in_avail() const         { return size(); }
 
     void SetType(int n)          { nType = n; }