.gitlint: de-duplicate defaults with zephyr_commit_rules.py

We don't need to have two different sets of Zephyr-specific default
values overriding one another, it's confusing.

Note this commit makes NO functional change, the effective defaults stay
the same. It does however make it easier to change defaults in the
future.

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