[Cleanup] Remove the obsolete Prerender field trial.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 1938051c788b7b126ae5d16d0e93fa45dcaa5910..0242404d8a9bea4008596f93dbb013e78d9e39b3 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -920,9 +920,7 @@ const char kPrerenderFromOmniboxSwitchValueEnabled[] = "enabled";
 // are dispatched from <link rel=prefetch href=...> elements.
 const char kPrerenderMode[]                 = "prerender";
 // These are the values the kPrerenderMode switch may have, as in
-// "--prerender=auto".
-//   auto: Allow field trial selection for prerender.
-const char kPrerenderModeSwitchValueAuto[]  = "auto";
+// "--prerender=disabled".
 //   disabled: No prerendering.
 const char kPrerenderModeSwitchValueDisabled[] = "disabled";
 //   enabled: Prerendering.