Remove trace of MouseEventPreconnect field trial.

The MouseEventPreconnect field trial has ended, but there is still a
trace of it remaining in ChromeBrowserFieldTrials. Remove it.

BUG=240959
TEST=chrome compiles

Review URL: https://codereview.chromium.org/1037033002

Cr-Commit-Position: refs/heads/master@{#322431}

chrome/browser/chrome_browser_field_trials.cc

@@ -47,7 +47,6 @@ void ChromeBrowserFieldTrials::InstantiateDynamicTrials() {
   // The following trials are used from renderer process.
   // Mark here so they will be sync-ed.
   base::FieldTrialList::FindValue("CLD1VsCLD2");
-  base::FieldTrialList::FindValue("MouseEventPreconnect");
   base::FieldTrialList::FindValue("DisplayList2dCanvas");
   // Activate the autocomplete dynamic field trials.
   OmniboxFieldTrial::ActivateDynamicTrials();