The low variable has not been used since it was added in d168b66682
and can be safely removed. The variable is present in the Sedgewick
paper "Analysis of Shellsort and Related Algorithms" as a parameter
to the shellsort function, but our implementation does not use it.
Remove to improve readability of the code.
Author: Koki Nakamura <btnakamurakoukil@oss.nttdata.com>
Discussion: https://postgr.es/m/8aeb7b3eda53ca4c65fbacf8f43628fb@oss.nttdata.com
1e37cc6e2c