iwx: Remove debug logging code

This debugging code was added this log as part of an effort to diagnose
firmware crashes in an earlier version of the driver.
It hasn't been used for a while and I'm not sure how useful it is for
this compared to other methods.

While reorganisation move strings for print mappings into if_iwx_debug.c
to reduce their scope.

Suggested by: bz
Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49783