Removed outdated tree parameters from sheriff-o-matic.

Now that the flakiness dashboard doesn't support a group parameter,
we don't need to pass the tree around in a bunch of places just to
compute the url to the dashboard.

This is the first step in getting the default UI for sheriff-o-matic
to be showing all the alerts for all the trees.

NOTRY=true
R=pdr@chromium.org,leviw@chromium.org
BUG=403133

Review URL: https://codereview.chromium.org/592653002

git-svn-id: svn://svn.chromium.org/blink/trunk@182446 bbb929c8-8fbe-4397-9dbb-9b2b20218538