Remove unused ParamEncoder module (#145)

ParamEncoder was required but never called anywhere in the gem.
Dead code — removing to reduce maintenance surface.

Author: hownowstephen

lib/customerio.rb

@@ -13,5 +13,4 @@ module Customerio
   require "customerio/requests/send_in_app_request"
   require "customerio/requests/trigger_broadcast_request"
   require "customerio/api"
-  require "customerio/param_encoder"
 end