Remove Usage.increment calls that have been shown to in fact be used (#1645)

I think these were added for undocumented features to see if they were
actually used and they were being used.

lib/liquid/forloop_drop.rb

@@ -30,10 +30,7 @@ module Liquid
     # @liquid_return [forloop]
     attr_reader :parentloop
 
-    def name
-      Usage.increment('forloop_drop_name')
-      @name
-    end
+    attr_reader :name
 
     # @liquid_public_docs
     # @liquid_summary

lib/liquid/tags/for.rb

@@ -177,7 +177,6 @@ module Liquid
       case key
       when 'offset'
         @from = if expr == 'continue'
-          Usage.increment('for_offset_continue')
           :continue
         else
           parse_expression(expr)