Revert "Cache tags_text to avoid computing tags each time when logging"

This reverts commit 05060ddba1.

Tags are per-fiber, so they can't be cached in an instance variable.

activesupport/lib/active_support/tagged_logging.rb

@@ -40,7 +40,6 @@ module ActiveSupport
       end
 
       def push_tags(*tags)
-        @tags_text = nil
         tags.flatten!
         tags.reject!(&:blank?)
         current_tags.concat tags
@@ -48,12 +47,10 @@ module ActiveSupport
       end
 
       def pop_tags(size = 1)
-        @tags_text = nil
         current_tags.pop size
       end
 
       def clear_tags!
-        @tags_text = nil
         current_tags.clear
       end
 
@@ -64,7 +61,6 @@ module ActiveSupport
       end
 
       def tags_text
-        @tags_text ||= begin
         tags = current_tags
         if tags.one?
           "[#{tags[0]}] "
@@ -73,7 +69,6 @@ module ActiveSupport
         end
       end
     end
-    end
 
     module LocalTagStorage # :nodoc:
       attr_accessor :current_tags