Drop reliance on GH-specific darkmode logic (jenkinsci#7920)

Author: NotMyFault

README.md

@@ -1,8 +1,5 @@
 <a href="https://jenkins.io">
-  <picture>
-    <source width="400" media="(prefers-color-scheme: dark)" srcset="https://www.jenkins.io/images/jenkins-logo-title-dark.svg">
-    <img width="400" src="https://www.jenkins.io/images/jenkins-logo-title.svg">
-  </picture>
+    <img width="400" src="https://www.jenkins.io/images/jenkins-logo-title-dark.svg">
 </a>
 
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