Removing explain support warning from the Railtie

This is causing a regression since the Active Record Railtie is trying to
connect to the development database in the application boot.

See #6197 (comment)

activerecord/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## unreleased ##
 
+*   Removed warning when `auto_explain_threshold_in_seconds` is set and the
+    connection adapter doesn't support explain.
+    This is causing a regression since the Active Record Railtie is trying to
+    connect to the development database in the application boot.
+
+    *Rafael Mendonça França*
+
 *   Do not reset `inheritance_column` when it's set explicitly.
     Backport of #5327.
 

activerecord/lib/active_record/railtie.rb

@@ -83,13 +83,6 @@ class Railtie < Rails::Railtie
       end
     end
 
-    initializer "active_record.validate_explain_support" do |app|
-      if app.config.active_record[:auto_explain_threshold_in_seconds] &&
-        !ActiveRecord::Base.connection.supports_explain?
-        warn "auto_explain_threshold_in_seconds is set but will be ignored because your adapter does not support this feature. Please unset the configuration to avoid this warning."
-      end
-    end
-
     # Expose database runtime to controller for logging.
     initializer "active_record.log_runtime" do |app|
       require "active_record/railties/controller_runtime"