Revert "Don't try to EXPLAIN select_db calls"

Reason: This was backported, but the test does not pass as is.

This reverts commit a33d320cf4.

activerecord/lib/active_record/explain_subscriber.rb

@@ -15,7 +15,7 @@ module ActiveRecord
     # On the other hand, we want to monitor the performance of our real database
     # queries, not the performance of the access to the query cache.
     IGNORED_PAYLOADS = %w(SCHEMA EXPLAIN CACHE)
-    EXPLAINED_SQLS = /\A\s*(select|update|delete|insert)\b/i
+    EXPLAINED_SQLS = /\A\s*(select|update|delete|insert)/i
     def ignore_payload?(payload)
       payload[:exception] || IGNORED_PAYLOADS.include?(payload[:name]) || payload[:sql] !~ EXPLAINED_SQLS
     end