Remove unused ParamEncoder module

[hownowstephen]

Summary

• ParamEncoder was required in customerio.rb but had zero internal callers anywhere in the gem
• Removed file and require line — 64 lines of dead code deleted

Test plan

• Grepped entire repo confirming no callers
• All 130 examples pass
• RuboCop clean

---

Note

Low Risk
Low risk: this deletes dead code (Customerio::ParamEncoder) and removes its require, with no remaining internal references found.

Overview
Removes the unused Customerio::ParamEncoder implementation and stops requiring it from lib/customerio.rb, reducing dead code in the gem without changing any active APIs.

^{Reviewed by Cursor Bugbot for commit c30ff6e. Bugbot is set up for automated code reviews on this repo. Configure here.}