ci: drop clang workflow, use main twister flow instead

Twister supports building with multiple toolchains in the same workflow
already, so there is no need for doing this in own workflow.

This will allow us to have a few targeted tests that are built with
multiple toolchains and frees up resources spent on duplicated builds in
a second workflow.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>