service: Drop a redundant 'const'

This was generating a warning from gcc.
3 jobs for master in 3 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #73317
ubuntu-16.04

00:01:50

passed #73318
ubuntu-17.10

00:03:12

passed #73319
ubuntu-18.04

00:02:52