revlog: stop using `atomictmp` for the split revlog

Since we already manually deal with writing on the side and delaying visibily,
we no longer need this.

mercurial/revlog.py

@@ -772,7 +772,6 b' class _InnerRevlog:'
             self.index_file,
             mode=b"w",
             checkambig=self.data_config.check_ambig,
-            atomictemp=True,
         )
 
     def split_inline(self, tr, header, new_index_file_path=None):