test: remove dead code in the bookrace extension

This code is the remain of a previous version of the code. It is never ran, so
we can remove it.

tests/test-bookmarks-corner-case.t

@@ -132,8 +132,6 b' We build a server side extension for thi'
   >             raise error.Abort("race scenario timed out")
   >         time.sleep(0.1)
   >     return orig(self, repo)
-  > 
-  >     repo.__class__ = racedrepo
   > def uisetup(ui):
   >     extensions.wrapfunction(bookmarks.bmstore, '__init__', wrapinit)
   > def e():