[libc++] Don't use a ppa for GCC

Ubuntu noble provides all the required compilers, but the ppa doesn't
yet. Simply don't use the ppa to make it possible to update to GCC 14.

Author: philnik777

libcxx/utils/ci/Dockerfile

@@ -142,7 +142,6 @@ EOF
 
 # Install the most recent GCC, like clang install the previous version as a transition.
 RUN <<EOF
-  sudo add-apt-repository ppa:ubuntu-toolchain-r/test
   sudo apt-get update
   sudo apt-get install -y \
     gcc-$((GCC_LATEST_VERSION - 1)) \