[iwm] Remove deprecated scan API definitions.
- This removes deprecated scan API definitions, which have been unused since the upgrade to version 16 firmware in change r303327.
- Part of this change matches the header-file changes in Linux git commit 1f9403863c080478ad78247c89b018e95bdfb027.
- No functional change.
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7937