Fix CI after GHA Update to Ubuntu 24.04 (#1781)

Summary:
Pull Request resolved: #1781

This new image removed preinstalled libc++, and the ability to install old version of Clang Format we were installing.

This manually installs libc++ when setting up Clang jobs, and fully removes the clang-format validation job, since it wasn't correctly running before, and it's probably more a pain to ask people to run this than to just run Arcanist when importing a change.

Changelog: [Internal]

Reviewed By: joevilches

Differential Revision: D68455129

fbshipit-source-id: b5767be832b2b5d46db7e60e18b66823819ba15a

.github/actions/setup-cpp/action.yml

@@ -16,6 +16,7 @@ runs:
       if: ${{ inputs.toolchain == 'Clang' }}
       shell: bash
       run: |
+        sudo apt-get install -y libc++-dev libc++abi-dev
         echo "CC=/usr/bin/clang" >> $GITHUB_ENV
         echo "CXX=/usr/bin/clang++" >> $GITHUB_ENV
         echo "CXXFLAGS=-stdlib=libc++" >> $GITHUB_ENV

.github/workflows/validate-cpp.yml

@@ -97,13 +97,3 @@ jobs:
           cmake -S . -B build -G Ninja -D CMAKE_BUILD_TYPE=Release
           cmake --build build
         working-directory: capture
-
-  clang-format:
-    name: Format
-    runs-on: ubuntu-latest
-
-    steps:
-      - uses: actions/checkout@v3
-
-      - name: clang-format
-        uses: ./.github/actions/clang-format