Remove redundant --libexecdir=/usr/libexec

This is the default at debhelper compat level 12.
6 jobs for debian/master in 7 minutes and 24 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #224229
build

00:02:52

 
  Test
passed #224232
autopkgtest

00:03:16

passed #224233
blhc

00:02:06

passed #224231
lintian

00:01:41

passed #224234
piuparts

00:04:31

passed #224230
reprotest

00:03:20