ci: footprint: Remove stale clang-related variables

This commit removes stale clang-related variables that are not used by
the footprint workflow.

Signed-off-by: Stephanos Ioannidis <[email protected]>
(cherry picked from commit ba14a4e)

Author: stephanosio

.github/workflows/footprint.yml

@@ -17,7 +17,6 @@ jobs:
       fail-fast: false
     env:
       ZEPHYR_SDK_INSTALL_DIR: /opt/toolchains/zephyr-sdk-0.15.0
-      CLANG_ROOT_DIR: /usr/lib/llvm-12
       ZEPHYR_TOOLCHAIN_VARIANT: zephyr
     steps:
       - name: Apply container owner mismatch workaround