chore(README): remove the second Scarf pixel — analytics consolidated to certctl.io

The README has carried two Scarf pixels for some time:
  - 89db181e-76e0-45cc-b9c0-790c3dfdfc73 (kept earlier as 'GitHub
    traffic complement to GitHub Insights')
  - b9379aff-9e5c-4d01-8f2d-9e4ffa09d126 (moved to the certctl.io
    landing page in commit ef7da23)

Re-evaluating: GitHub Insights → Traffic already provides repo
views, uniques, clones, and referring sites with click counts at
higher granularity than a Scarf pixel can extract from the README
(Scarf can only see 'github.com' as the referrer; GitHub Insights
knows the actual external referrer that landed the visitor on the
README). The 89db181e pixel was duplicative-and-worse.

Removing it. All certctl analytics now consolidate to:
  - GitHub Insights → Traffic (built-in, more granular than Scarf
    on the README surface)
  - certctl.io's b9379aff pixel (referrer-attribution for landing-
    page traffic, where Scarf actually adds value)
  - Scarf Docker Gateway via shankar0123.docker.scarf.sh/* (when
    the Helm chart + docker-compose.yml are routed through it —
    follow-up work)

The Docker-pull example block at line 246 stays (it documents
how operators install certctl via the Scarf gateway). Only the
in-README tracking <img> is removed.

README.md

@@ -2,8 +2,6 @@
   <img src="docs/screenshots/logo/certctl-logo.png" alt="certctl logo" width="450">
 </p>
 
-<img referrerpolicy="no-referrer-when-downgrade" src="https://static.scarf.sh/a.png?x-pxid=89db181e-76e0-45cc-b9c0-790c3dfdfc73" />
-
 # certctl — Self-Hosted Certificate Lifecycle Platform
 
 [![License](https://img.shields.io/badge/license-BSL%201.1-blue.svg)](LICENSE)