MFC r344024:

CID 1009492: Logically dead code in sys/cam/scsi/scsi_xpt.c

In probedone(), for the PROBE_REPORT_LUNS case, all paths that
fall to the bottom of the case set lp to NULL, so the test for a
non-NULL value of lp and call to free() if true is dead code as
the test can never be true. Fix by eliminating the whole if
statement. To guard against a possible future change that accidentally
violates this assumption, use a KASSERT() to catch if lp is
non-NULL.

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