Revert "gh-129005: _pyio.BufferedIO remove copy on readall (#129454)" (#129500)

This reverts commit e1c4ba9.

Author: cmaloney

Lib/_pyio.py

@@ -1062,9 +1062,6 @@ def _read_unlocked(self, n=None):
                 if chunk is None:
                     return buf[pos:] or None
                 else:
-                    # Avoid slice + copy if there is no data in buf
-                    if not buf:
-                        return chunk
                     return buf[pos:] + chunk
             chunks = [buf[pos:]]  # Strip the consumed bytes.
             current_size = 0