west : runners : Fix error message when runners.yaml is not found.

Adjust error message so that it clearly states runners.yaml is
missing from <build_dir>/zephyr, instead of referencing CMake cache
variable ZEPHYR_RUNNERS_YAML, which is no longer used (since
3124c029 ).
Also clean up that variable in CMake since it is no longer used
(0 other references in entire tree).
Fixes #70605

Signed-off-by: Louis Feller <louis.feller@st.com>