Remove unused nested_form gem

Gemfile

@@ -333,9 +333,6 @@ gem 'email_reply_parser', '~> 0.5.8'
 gem 'ruby-prof', '~> 0.16.2'
-## CI
-gem 'nested_form', '~> 0.3.2'
-
 # OAuth
 gem 'oauth2', '~> 1.2.0'

Gemfile.lock

@@ -410,7 +410,6 @@ GEM
     multi_xml (0.5.5)
     multipart-post (2.0.0)
     mysql2 (0.3.20)
-    nested_form (0.3.2)
     net-ldap (0.12.1)
     net-ssh (3.0.1)
     newrelic_rpm (3.16.0.318)
@@ -894,7 +893,6 @@ DEPENDENCIES
   minitest (~> 5.7.0)
   mousetrap-rails (~> 1.4.6)
   mysql2 (~> 0.3.16)
-  nested_form (~> 0.3.2)
   net-ssh (~> 3.0.1)
   newrelic_rpm (~> 3.16)
   nokogiri (~> 1.6.7, >= 1.6.7.2)