contrib: Remove optimize-pngs.py script, which lives in the maintainer repo

https://github.com/bitcoin-core/bitcoin-maintainer-tools/blob/master/optimize-pngs.py

Author: MarcoFalke

contrib/devtools/README.md

@@ -89,12 +89,6 @@ example:
 BUILDDIR=$PWD/build contrib/devtools/gen-manpages.sh
 ```
 
-optimize-pngs.py
-================
-
-A script to optimize png files in the bitcoin
-repository (requires pngcrush).
-
 security-check.py and test-security-check.py
 ============================================