Remove a write-only variable.

While refactoring an earlier series of changes during review, the
'saved_data' variable stopped being used at the bottom of if_ioctl().

Suggested by: brooks
Reviewed by: brooks, imp, kib
Fixes: d17e0940f79f Rework compat shims in ifioctl().
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D30197