Partially revert "READMEs: prevent clickable logo on GitHub"

Logo becomes invisible on pub.dev.

Author: greenrobot-team

README.md

@@ -1,7 +1,5 @@
 <p align="center">
-  <picture>
-    <img src="https://raw.githubusercontent.com/objectbox/objectbox-dart/main/.github/logo.png" alt="ObjectBox" width="400px">
-  </picture>
+  <img src="https://raw.githubusercontent.com/objectbox/objectbox-dart/main/.github/logo.png" alt="ObjectBox" width="400px">
 </p>
 
 <p align="center">