streams: remove AutoFile::Get() entirely

Co-Authored-By: David Gumberg <davidzgumberg@gmail.com>

Github-Pull: bitcoin/bitcoin#30884
Rebased-From: a240e150e8

src/test/fuzz/autofile.cpp

@@ -56,7 +56,6 @@ FUZZ_TARGET(autofile)
                 WriteToStream(fuzzed_data_provider, auto_file);
             });
     }
-    (void)auto_file.Get();
     (void)auto_file.IsNull();
     if (fuzzed_data_provider.ConsumeBool()) {
         FILE* f = auto_file.release();