Remove browser.Start

BUG=417528
TEST=bots

Review URL: https://codereview.chromium.org/604423006

Cr-Commit-Position: refs/heads/master@{#297205}

tools/telemetry/telemetry/core/browser.py

@@ -348,8 +348,3 @@ class Browser(object):
 
        See the documentation of the SystemInfo class for more details."""
     return self._browser_backend.GetSystemInfo()
-
-  # TODO: Remove after call to Start() has been removed from
-  # related authotest files.
-  def Start(self):
-    pass