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/bulk_import/discourse_merger.rb

@@ -4,7 +4,7 @@ require_relative "base"
 
 class BulkImport::DiscourseMerger < BulkImport::Base
 
-  NOW ||= "now()".freeze
+  NOW ||= "now()"
   CUSTOM_FIELDS = ['category', 'group', 'post', 'topic', 'user']
 
   # DB_NAME: name of database being merged into the current local db