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)