Drop redundant pip self-upgrade in CI

Ubuntu 24.04 ships pip 24.0, which already handles the TF 2.21 and
Keras 3.14 wheels. The self-upgrade step also runs into PEP 668's
RECORD-file check on apt-installed pip, so removing it both simplifies
setup and avoids that quirk.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Dobiasd

.github/workflows/ci.yml

@@ -12,7 +12,6 @@ jobs:
           # system libs
           sudo apt-get install libblas-dev liblapack-dev libatlas-base-dev gfortran
           # python libs
-          sudo pip3 install --break-system-packages --upgrade pip
           sudo pip3 install --break-system-packages numpy scipy h5py "tensorflow==2.21.0" "keras==3.14.0"
           echo "Python version:"
           python3 --version