Remove uses of string.mb_chars.downcase.

`string.mb_chars.downcase` was used to correctly downcase Unicode
characters when downcasing strings in Ruby <2.4. This hasn't been needed
since Ruby 2.4.

app/models/pool_version.rb

@@ -91,7 +91,7 @@ class PoolVersion < ApplicationRecord
   end
 
   def self.normalize_name_for_search(name)
-    normalize_name(name).mb_chars.downcase
+    normalize_name(name).downcase
   end
 
   def previous