gems: remove whenever gem.

We're now using the `clockwork` gem for cronjobs in production. See
config/initializers/clockwork.rb.

Gemfile.lock

@@ -140,7 +140,6 @@ GEM
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
     childprocess (3.0.0)
-    chronic (0.10.2)
     clockwork (2.0.4)
       activesupport
       tzinfo
@@ -503,8 +502,6 @@ GEM
     websocket-driver (0.7.5)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.5)
-    whenever (1.0.0)
-      chronic (>= 0.6.3)
     xpath (3.2.0)
       nokogiri (~> 1.8)
     yard (0.9.26)
@@ -598,7 +595,6 @@ DEPENDENCIES
   unicorn-worker-killer
   view_component
   webpacker (= 6.0.0.beta.6)
-  whenever
 
 BUNDLED WITH
    2.1.4