po: Remove old Autotools file

The `Makevars` file is only meant to be used by `make`.
4 jobs for master in 4 minutes and 21 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Test
passed build_stable #1642417

00:04:20

passed flawfinder-sast #1642414

00:00:16

passed secret_detection #1642416

00:00:30

passed semgrep-sast #1642415

00:00:40