intel_adsp: remove rimage sign() from `west flash`

`west sign` has been invoked by `west build` (through CMake) since
commit fad2da39, almost one year ago. During that time, this new
workflow has been refined and successfully used by at least two vendors,
multiple CIs across both SOF and Zephyr and many developers.

At the time, the ability to sign from `west flash` was preserved for
backwards compatibility. This means rimage parameters can come from many
different places at once and that rimage can be invoked twice during a
single `west flash` invocation!

Now that Zephyr 3.5 has been released, we need to reduce the number of
rimage use cases and the corresponding validation complexity and
maintenance workload to simplify and accelerate new features like
splitting rimage configuration files (#65411)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>