quicksort: Remove "switch to insertion sort" optimization

Reference commit from NetBSD:
http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/stdlib/qsort.c?rev=1.20&content-type=text/x-cvsweb-markup&only_with_tag=MAIN

Change-Id: I4ce2507fa5f3cc240d478f84a1e59bc18f74eb8a
Reviewed-on: http://review.couchbase.org/36459
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>