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/artist.rb

@@ -64,7 +64,7 @@ class Artist < ApplicationRecord
   concerning :NameMethods do
     class_methods do
       def normalize_name(name)
-        name.to_s.mb_chars.downcase.strip.gsub(/ /, "_").to_s
+        name.to_s.downcase.strip.gsub(/ /, "_").to_s
       end
 
       def normalize_other_names(other_names)

app/models/pool.rb

@@ -86,7 +86,7 @@ class Pool < ApplicationRecord
   end
 
   def self.normalize_name_for_search(name)
-    normalize_name(name).mb_chars.downcase
+    normalize_name(name).downcase
   end
 
   def self.named(name)

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

app/models/tag.rb

@@ -173,7 +173,7 @@ class Tag < ApplicationRecord
 
     class_methods do
       def normalize_name(name)
-        name.to_s.mb_chars.downcase.strip.tr(" ", "_").to_s
+        name.to_s.downcase.strip.tr(" ", "_").to_s
       end
 
       def create_for_list(names)

app/models/tag_relationship.rb

@@ -33,8 +33,8 @@ class TagRelationship < ApplicationRecord
   validate :antecedent_and_consequent_are_different
 
   def normalize_names
-    self.antecedent_name = antecedent_name.mb_chars.downcase.tr(" ", "_")
-    self.consequent_name = consequent_name.mb_chars.downcase.tr(" ", "_")
+    self.antecedent_name = antecedent_name.downcase.tr(" ", "_")
+    self.consequent_name = consequent_name.downcase.tr(" ", "_")
   end
 
   def is_rejected?

app/models/user.rb

@@ -187,7 +187,7 @@ class User < ApplicationRecord
       end
 
       def normalize_name(name)
-        name.to_s.mb_chars.downcase.strip.tr(" ", "_").to_s
+        name.to_s.downcase.strip.tr(" ", "_").to_s
       end
     end