revlog: drop local assignment of cache variable

The purpose of this code was to provide thread safety. With the
conversion of hgweb to use separate localrepository instances per
request/thread, we should no longer have any consumers that need to
access revlog instances from multiple threads. Remove the code.

mercurial/revlog.py

@@ -1049,14 +1049,13 b' class revlog(object):'
             node = nodeorrev
             rev = None
 
-        _cache = self._cache # grab local copy of cache to avoid thread race
         cachedrev = None
         if node == nullid:
             return ""
-        if _cache:
-            if _cache[0] == node:
-                return _cache[2]
-            cachedrev = _cache[1]
+        if self._cache:
+            if self._cache[0] == node:
+                return self._cache[2]
+            cachedrev = self._cache[1]
 
         # look up what we need to read
         text = None
@@ -1084,7 +1083,7 b' class revlog(object):'
 
         if iterrev == cachedrev:
             # cache hit
-            text = _cache[2]
+            text = self._cache[2]
         else:
             chain.append(iterrev)
         chain.reverse()