doc: release process: stop creating zephyr- prefixed tags

Drop the process step to create zephyr- prefixed tags.

This has been done so that autogenerated release files have a sensible
name, but GitHub started to automatically prefix the project names to
release artifacts anyway using HTTP headers, which now results in file
named like: zephyr-zephyr-v3.2.0.tar.gz, so the extra tags are now
causing the opposite problem.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>