fix: delete boilerplate favicon on login page

Previously, network tools show a 404 getting
/users/assets/images/favicon.svg on /users/sign_in, and some tests
intermittently failed with this 404. Delete the extraneous favicon link
from devise boilerplate to let the normal favicon work on the login
screen.

Fixes test flake at
https://github.com/rubyforgood/casa/wiki/Flaky-tests#suspected-testing-tool-crash

Author: willmurphyscode

app/views/layouts/devise.html.erb

@@ -4,10 +4,6 @@
   <meta charset="UTF-8">
   <meta http-equiv="X-UA-Compatible" content="IE=edge">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
-  <link
-    rel="shortcut icon"
-    href="assets/images/favicon.svg"
-    type="image/x-icon">
   <title>CASA Volunteer Tracker</title>
 
   <%= javascript_include_tag "application", "data-turbo-track": "reload", defer: true %>