libkern: Drop incorrect qsort optimization

See 5205b32de3fb for details.

PR: 287089
MFC after: 1 week
Reviewed by: jlduran
Differential Revision: https://reviews.freebsd.org/D51919

(cherry picked from commit ef8f3e913156aa268e07ae1daa68e3bc3f1c4d29)