logger: remove redundant opt_str_freeze use

Strings in "when" statements are allocation-free, so there's
never any reason to uglify the code to use opt_str_freeze over
the normal putobject instructions, here.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58076 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/logger.rb

@@ -268,17 +268,17 @@ class Logger
       @level = severity
     else
       case severity.to_s.downcase
-      when 'debug'.freeze
+      when 'debug'
         @level = DEBUG
-      when 'info'.freeze
+      when 'info'
         @level = INFO
-      when 'warn'.freeze
+      when 'warn'
         @level = WARN
-      when 'error'.freeze
+      when 'error'
         @level = ERROR
-      when 'fatal'.freeze
+      when 'fatal'
         @level = FATAL
-      when 'unknown'.freeze
+      when 'unknown'
         @level = UNKNOWN
       else
         raise ArgumentError, "invalid log level: #{severity}"