DEV: Drop our `mail` gem fork (#16622)

Bianca's patch was finally released (in rc release for now, but still!)

Gemfile

@@ -41,7 +41,7 @@ gem 'actionview_precompiler', require: false
 
 gem 'seed-fu'
 
-gem 'mail', git: 'https://github.com/discourse/mail.git', require: false
+gem 'mail', '2.8.0.rc1', require: false
 gem 'mini_mime'
 gem 'mini_suffix'
 

Gemfile.lock

@@ -1,10 +1,3 @@
-GIT
-  remote: https://github.com/discourse/mail.git
-  revision: 5b700fc95ee66378e0cf2559abc73c8bc3062a4b
-  specs:
-    mail (2.8.0.edge)
-      mini_mime (>= 0.1.1)
-
 GEM
   remote: https://rubygems.org/
   specs:
@@ -228,6 +221,11 @@ GEM
       nokogiri (>= 1.5.9)
     lru_redux (1.1.0)
     lz4-ruby (0.3.3)
+    mail (2.8.0.rc1)
+      mini_mime (>= 0.1.1)
+      net-imap
+      net-pop
+      net-smtp
     maxminddb (0.1.22)
     memory_profiler (1.0.0)
     message_bus (4.2.0)
@@ -566,7 +564,7 @@ DEPENDENCIES
   loofah
   lru_redux
   lz4-ruby
-  mail!
+  mail (= 2.8.0.rc1)
   maxminddb
   memory_profiler
   message_bus