Merge #18885: contrib: Move optimize-pngs.py script to the maintainer repo

fa13090 contrib: Remove optimize-pngs.py script, which lives in the maintainer repo (MarcoFalke)

Pull request description:

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

  Bitcoin Core should focus on the full node implementation, not on scripts to compress png images.

  This script is only used when new PNG files are added to the repo. This happens about once every two years. So fetching the script from the other repo should not be a burden, but removing it from this repo is going to cut down on the meta files we need to maintain in the main repo.

ACKs for top commit:
  practicalswift:
    ACK fa13090 -- `+0 lines, -82 lines` :)
  promag:
    ACK fa13090.
  hebasto:
    ACK fa13090, verified that script is already [moved](bitcoin-core/bitcoin-maintainer-tools#56).

Tree-SHA512: 37d111adae769bcddc6ae88041032d5a2b8b228fec67f555c8333c38de3992f5138b30bea868d7d6d6b7f966a47133e5853134373b149ab23cba3b8b560ecb31

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
 ============================================