DEV: stop freezing frozen strings

We have the `# frozen_string_literal: true` comment on all our
files. This means all string literals are frozen. There is no need
to call #freeze on any literals.

For files with `# frozen_string_literal: true`

```
puts %w{a b}[0].frozen?
=> true

puts "hi".frozen?
=> true

puts "a #{1} b".frozen?
=> true

puts ("a " + "b").frozen?
=> false

puts (-("a " + "b")).frozen?
=> true
```

For more details see: https://samsaffron.com/archive/2018/02/16/reducing-string-duplication-in-ruby

script/import_scripts/mbox/support/indexer.rb

@@ -39,7 +39,7 @@ module ImportScripts::Mbox
 
     private
 
-    METADATA_FILENAME = 'metadata.yml'.freeze
+    METADATA_FILENAME = 'metadata.yml'
     IGNORED_FILE_EXTENSIONS = ['.dbindex', '.dbnames', '.digest', '.subjects', '.yml']
 
     def index_category(directory)