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

lib/migration/base_dropper.rb

@@ -2,7 +2,7 @@
 
 module Migration
   class BaseDropper
-    FUNCTION_SCHEMA_NAME ||= "discourse_functions".freeze
+    FUNCTION_SCHEMA_NAME ||= "discourse_functions"
 
     def self.create_readonly_function(table_name, column_name = nil)
       DB.exec <<~SQL